Storage Battery, Battery Control Unit, and Electronic Device

ABSTRACT

A storage battery includes positive and negative electrodes and an electrolytic solution. The negative electrode includes a first element and carbon. The first element is any of silicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium. The negative electrode includes an active material and a first layer in contact with a surface of the active material. The first layer has a thickness of 10 nm to 1000 nm inclusive. The electrolytic solution contains first and second cations. The first cation is one or more of a lithium ion, a sodium ion, a calcium ion, and a magnesium ion. The second cation is an imidazolium cation or a tertiary sulfonium cation.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, one embodimentof the present invention relates to a semiconductor device, a displaydevice, a light-emitting device, a power storage device, a storagedevice, a driving method thereof, or a manufacturing method thereof. Inparticular, one embodiment of the present invention relates to a powerstorage device and a manufacturing method thereof.

Note that a power storage device in this specification refers to everyelement and/or device having a function of storing electric power.

2. Description of the Related Art

In recent years, a variety of power storage devices, for example,secondary batteries such as lithium-ion secondary batteries, lithium-ioncapacitors, and air cells have been actively developed. In particular,demand for lithium-ion secondary batteries with a high output and a highenergy density has rapidly grown with the development of thesemiconductor industry, for electronic devices, for example, portableinformation terminals such as mobile phones, smartphones, and laptopcomputers, portable music players, and digital cameras; medicalequipment; next-generation clean energy vehicles such as hybrid electricvehicles (HEVs), electric vehicles (EVs), and plug-in hybrid electricvehicles (PHEVs); and the like. The lithium-ion secondary batteries areessential as rechargeable energy supply sources for today's informationsociety.

In a storage battery that utilizes the reaction of carrier ions, such asa lithium-ion battery, the volume of an active material might be changedby charge and discharge process. For example, it is known that theinterlayer distance of graphite increases from 0.336 nm to 0.370 nm asdisclosed in Non-patent Document 1 (see Non-Patent Document 1, pp.333-334).

As disclosed in Patent Document 1, for example, the shape or volume ofan alloy-based material such as silicon might be changed by repeatedcharge and discharge cycles.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2013-214501-   [Non-Patent Document 1] Masaki Yoshio et al., “Lithium-Ion Batteries    Science and Technologies”, Springer, chapter 16, pp. 333-334

SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel electrode. Another object of one embodiment of the presentinvention is to provide a novel power storage device.

Another object of one embodiment of the present invention is to providea power storage device in which a decrease in capacity caused by chargeand discharge cycles is small. Another object of one embodiment of thepresent invention is to provide a long-life power storage device.Another object of one embodiment of the present invention is to providea highly reliable power storage device.

Another object of one embodiment of the present invention is to providean electrode with a high capacity. Another object of one embodiment ofthe present invention is to provide a power storage device with highenergy density. Another object of one embodiment of the presentinvention is to provide a power storage device whose characteristicssuffer little degradation when external force is repeatedly applied tothe power storage device.

Note that the description of these objects does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention is a storage battery including apositive electrode, a negative electrode, and an electrolytic solution.The negative electrode includes a first element and carbon. The firstelement is any of silicon, tin, gallium, aluminum, germanium, lead,antimony, bismuth, silver, zinc, cadmium, and indium. The negativeelectrode includes an active material and a first layer in contact witha surface of the active material. The first layer includes a portionwith a thickness larger than or equal to 10 nm and smaller than or equalto 1000 nm. The electrolytic solution contains a first cation and asecond cation. The first cation is one or more of a lithium ion, asodium ion, a calcium ion, and a magnesium ion. The second cation is animidazolium cation or a tertiary sulfonium cation.

Another embodiment of the present invention is a storage batteryincluding a positive electrode, a negative electrode, and anelectrolytic solution. The negative electrode includes a first elementand carbon. The first element is any of silicon, tin, gallium, aluminum,germanium, lead, antimony, bismuth, silver, zinc, cadmium, and indium.The negative electrode includes a first region, a second region incontact with a surface of the first region, and a third region incontact with a surface of the second region. The second region and thethird region each have a shape of a layer. The thickness of the secondregion is larger than or equal to 10 nm and smaller than or equal to 500nm. The thickness of the third region is larger than or equal to 10 nmand smaller than or equal to 1000 nm. The atomic ratio of carbon to thefirst element in the first region is x₁:y₁. The atomic ratio of carbonto the first element in the second region is x₂:y₂. The atomic ratio ofcarbon to the first element in the third region is x₃:y₃. x₁/y₁ issmaller than or equal to 3. x₂/y₂ is larger than or equal to 0.1 andsmaller than 10. x₃/y₃ is larger than or equal to 5. The electrolyticsolution contains a first cation and a second cation. The first cationis one or more of a lithium ion, a sodium ion, a calcium ion, and amagnesium ion. The second cation is an imidazolium cation or a tertiarysulfonium cation.

In the above structure, an aromatic cation, an aliphatic onium cation,or the like can be used as the second cation. Examples of the aromaticcation include a pyridinium cation and an imidazolium cation. Examplesof the aliphatic onium cation include a quaternary ammonium cation, atertiary sulfonium cation, and a quaternary phosphonium cation.Furthermore, in the above structure, any of the cations represented byChemical Formulas (1) to (17) described below is preferably used as thesecond cation.

In the above structure, as an anion, a monovalent amide-based anion, amonovalent methide-based anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, a tetrafluoroborate anion, aperfluoroalkylborate anion, a hexafluorophosphate anion, aperfluoroalkylphosphate anion, or the like can be used.

Another embodiment of the present invention is an electronic deviceincluding the storage battery described in any of the above. Theelectronic device preferably includes a display device. It is preferredthat the electronic device include an input-output terminal and theinput-output terminal have a function of performing wirelesscommunication.

Another embodiment of the present invention is a battery control unitincluding the storage battery described in any of the above.

One embodiment of the present invention can provide a novel electrode.Another embodiment of the present invention can provide a novel powerstorage device.

Another embodiment of the present invention can provide a power storagedevice in which a decrease in capacity with an increasing number ofcharge and discharge cycles is small. Another embodiment of the presentinvention can provide a long-life power storage device. Anotherembodiment of the present invention can provide a highly reliable powerstorage device.

Another embodiment of the present invention can provide an electrodewith a high capacity. Another embodiment of the present invention canprovide a power storage device with high energy density. Anotherembodiment of the present invention can provide a power storage devicewhose characteristics suffer little degradation when external force isrepeatedly applied to the power storage device.

Note that the descriptions of these effects do not disturb the existenceof other effects. One embodiment of the present invention does notnecessarily have all the effects listed above. Other effects will beapparent from and can be derived from the descriptions of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Accompanying Drawings:

FIGS. 1A to 1E each illustrate constituents of an electrolytic solution;

FIGS. 2A and 2B each illustrate a particle of one embodiment of thepresent invention;

FIGS. 3A and 3B are diagrams illustrating part of a cross section of anelectrode;

FIG. 4 illustrates a storage battery;

FIGS. 5A and 5B are each a cross-sectional view of a storage battery;

FIGS. 6A and 6B illustrate a method for fabricating a storage battery;

FIGS. 7A and 7B illustrate a method for fabricating a storage battery;

FIG. 8 illustrates a storage battery;

FIGS. 9A and 9B illustrate a method for fabricating a storage battery;

FIGS. 10A and 10B illustrate a method for fabricating a storage battery;

FIGS. 11A to 11C illustrate a method for fabricating a storage battery;

FIGS. 12A to 12C illustrate a method for fabricating a storage battery;

FIGS. 13A to 13C illustrate the curvature radius of a surface;

FIGS. 14A to 14D illustrate the curvature radius of a film;

FIGS. 15A and 15B illustrate a coin-type storage battery;

FIGS. 16A and 16B illustrate a cylindrical storage battery;

FIGS. 17A to 17C are each a part of a cross-sectional view of a storagebattery;

FIGS. 18A and 18B are each a part of a cross-sectional view of a storagebattery;

FIGS. 19A to 19C are parts of cross-sectional views of a storagebattery;

FIGS. 20A to 20C illustrate an example of a storage battery;

FIGS. 21A to 21C illustrate an example of a storage battery;

FIGS. 22A and 22B illustrate an example of a power storage system;

FIGS. 23A1, 23A2, 23B1, and 23B2 illustrate examples of power storagesystems;

FIGS. 24A and 24B illustrate an example of a power storage system;

FIGS. 25A to 25G illustrate examples of electronic devices;

FIGS. 26A to 26C illustrate an example of an electronic device;

FIG. 27 illustrates examples of electronic devices;

FIGS. 28A and 28B illustrate examples of electronic devices;

FIG. 29 is a block diagram illustrating one embodiment of the presentinvention;

FIGS. 30A to 30C are schematic views each illustrating one embodiment ofthe present invention;

FIG. 31 is a circuit diagram illustrating one embodiment of the presentinvention;

FIG. 32 is a circuit diagram illustrating one embodiment of the presentinvention;

FIGS. 33A to 33C are schematic views each illustrating one embodiment ofthe present invention;

FIG. 34 is a block diagram illustrating one embodiment of the presentinvention;

FIG. 35 is a flow chart showing one embodiment of the present invention;

FIGS. 36A and 36B each show CV measurement results;

FIGS. 37A and 37B each show CV measurement results;

FIGS. 38A and 38B each show CV measurement results;

FIG. 39 shows CV measurement results;

FIG. 40 shows CV measurement results;

FIGS. 41A and 41B each show CV measurement results;

FIGS. 42A and 42B each show charge and discharge characteristics;

FIG. 43 shows cycle performances;

FIGS. 44A and 44B each show XPS analysis results;

FIGS. 45A and 45B each show XPS analysis results;

FIG. 46 shows XPS analysis results;

FIG. 47 shows a cross-sectional TEM observation result;

FIG. 48 shows a cross-sectional TEM observation result;

FIG. 49A shows a cross-sectional observation image and FIG. 49B showsEDX analysis results;

FIG. 50A shows a cross-sectional observation image and FIG. 50B showsEDX analysis results;

FIGS. 51A and 51B each show EDX analysis results;

FIG. 52A shows a cross-sectional observation image and FIG. 52B showsEELS analysis results;

FIG. 53A shows a cross-sectional observation image and FIG. 53B showsEELS analysis results; and

FIGS. 54A and 54B illustrate a negative electrode of one embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the descriptions of theembodiments and examples and it is easily understood by those skilled inthe art that the mode and details can be changed variously. Accordingly,the present invention should not be interpreted as being limited to thedescriptions of the embodiments below.

Note that in drawings used in this specification, the sizes,thicknesses, and the like of components such as films, layers,substrates, and regions are exaggerated for simplicity in some cases.Therefore, the sizes of the components are not limited to the sizes inthe drawings and relative sizes between the components.

Note that the ordinal numbers such as “first” and “second” in thisspecification and the like are used for convenience and do not denotethe order of steps, the stacking order of layers, or the like.Therefore, for example, description can be made even when “first” isreplaced with “second” or “third”, as appropriate. In addition, theordinal numbers in this specification and the like are not necessarilythe same as those which specify one embodiment of the present invention.

Note that in structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions are denoted by common reference numerals in differentdrawings, and descriptions thereof are not repeated. Further, the samehatching pattern is applied to portions having similar functions, andthe portions are not especially denoted by reference numerals in somecases.

Note that in this specification and the like, a positive electrode and anegative electrode for a power storage device may be collectivelyreferred to as a power storage device electrode; in this case, the powerstorage device electrode refers to at least one of the positiveelectrode and the negative electrode for the power storage device.

Embodiment 1

In this embodiment, a storage battery of one embodiments of the presentinvention will be described.

A storage battery of one embodiment of the present invention includes apositive electrode, a negative electrode, and an electrolytic solution.

The electrolytic solution in the storage battery of one embodiment ofthe present invention preferably contains an ionic liquid. Furthermore,the storage battery of one embodiment of the present inventionpreferably includes first cations in addition to ions contained in theionic liquid. As the first cations, alkali metal ions, alkaline earthmetal ions, or the like can be used, for example. Alkali metal ions andalkaline earth metal ions serve as carrier ions of the storage battery.

Examples of alkali metals include lithium, sodium, and potassium.Examples of alkaline earth metals include calcium, strontium, barium,beryllium, and magnesium.

In the case of using lithium ions as carrier ions, one of lithium saltssuch as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN, LiBr, LiI, Li₂SO₄,Li₂B₁₀Cl₁₀, Li₂B₁₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃,LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂)(CF₃SO₂), and LiN(C₂F₅SO₂)₂ canbe dissolved in an ionic liquid, or two or more of these lithium saltscan be dissolved in an ionic liquid in an appropriate combination at anappropriate ratio.

Here, the concentration of carrier ions is preferably higher than 0.1mol/L and lower than 3 mol/L, more preferably higher than or equal to0.3 mol/L and lower than or equal to 2.5 mol/L.

The electrolytic solution may contain a solvent other than the ionicliquid, such as an aprotic organic solvent. As an aprotic organicsolvent, for example, one of ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate, chloroethylene carbonate, vinylenecarbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC),diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate,methyl acetate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

In charging and discharging the storage battery, a decompositionreaction of the electrolytic solution might occur. The electrolyticsolution is decomposed in the vicinity of a surface of an electrodemainly by an electrical reaction.

In the storage battery using alkali metal ions, alkaline earth metalions, or the like as carrier ions, for example, the reaction potentialof a negative electrode is low and thus a decomposition reaction of theelectrolytic solution easily occurs, in some cases. For example, thecase where the electrolytic solution is decomposed in the vicinity of asurface of the negative electrode by a reduction reaction will bedescribed. In the case where the decomposition of the electrolyticsolution is an irreversible reaction, irreversible capacity mightincrease. Note that irreversible capacity is a difference between chargecapacity and discharge capacity. An increase in irreversible capacitydecreases the capacity of the storage battery.

To inhibit a decrease in the capacity of the storage battery, it ispreferable to inhibit an irreversible reaction of the electrolyticsolution in the vicinity of the electrode surface. For example, it ispreferred that a reaction of main carrier ions, alkali metal ions oralkaline earth metal ions, of constituents of the electrolytic solutionbe promoted and reactions of other constituents such as cations andanions in the ionic liquid be inhibited.

FIG. 1A is a schematic view illustrating constituents of an electrolyticsolution near a surface of a negative electrode active material 671 of astorage battery. The electrolytic solution contains first cations 681and an ionic liquid. The ionic liquid contains cations 682 and anions683. For example, lithium ions can be used as the first cations 681, and1-ethyl-3-methylimidazolium (EMI) can be used as the cations 682.

Lithium has a significantly low redox potential, specifically, 3.045 Vlower than a standard electrode potential. The reaction potential of anegative electrode active material is preferably as low as possible, inwhich case the voltage of a power storage device can be high. On theother hand, when the potential is low, power of reducing an electrolytesolution is high, so that an organic solvent or the like in anelectrolyte solution might reductively decompose. The reaction potentialof a negative electrode of a lithium-ion battery is preferably equal toor slightly higher than the redox potential of lithium, in which casethe voltage of a power storage device can be high. Meanwhile, thereaction potential of cations in a solvent of an electrolytic solution,e.g., the ionic liquid is higher than the redox potential of lithium inmany cases.

Charging the storage battery is accompanied by a decrease in thepotential of the negative electrode active material 671. Chargeaccumulates in the negative electrode until the potential of thenegative electrode active material 671 reaches a potential at whichreactions of the first cations 681, the cations 682, and the anions 683noticeably occur. The first cations 681 preferably form an electricdouble layer on a surface of the negative electrode active material 671as illustrated in FIG. 1B with accumulation of the charge. The anions683 are arranged on a surface of the electric double layer formed by thefirst cations 681. In FIG. 1B, the cations 682 are prevented fromreaching the surface of the negative electrode active material 671.

In the case where the reaction potential of the cations 682 is higherthan that of the first cations 681, a noticeable reaction of the cations682 occurs when the potential of the negative electrode decreases to thereaction potential of the cations 682. In the case where the cations 682are prevented from reaching a surface of the negative electrode asillustrated in FIG. 1B, however, a reaction of the cations 682 can beinhibited until the potential of the negative electrode reaches thereaction potential of the first cations 681.

Next, the case where the potential of the negative electrode decreasesto a potential at which a noticeable reaction of the first cations 681occurs will be described. The first cations 681 are consumed by thereduction reaction of the first cations 681. The reduction of the firstcations 681 might be accompanied by formation of a compound of the firstcations 681 and the negative electrode active material 671.Alternatively, the reduction might unionize the first cations 681 toform a deposition layer 681 b on the surface of the negative electrodeactive material 671.

The electric double layer formed on the surface of the negativeelectrode active material 671 disappears with the consumption of thefirst cations 681, so that new cations reach the surface of the negativeelectrode active material 671. When the cations 682 reach the surface ofthe negative electrode active material 671, an irreversible reaction,e.g., reductive decomposition, of the cations 682 might occur to form areaction product 684. The reaction product 684 might be deposited toform a coating film 685 on the surface of the negative electrode activematerial 671.

Thus, the first cations 681 preferably reach the surface of the negativeelectrode active material 671 faster than the cations 682. For example,in the case where the diffusion rate of the first cations 681 in theelectrolytic solution is higher than that of the cations 682 asillustrated in FIG. 1D, a reaction of the cations 682 can be inhibitedand a reaction of the first cations 681 serving as main carrier ions canbe promoted.

Here, the case where an aprotic organic solvent is used as a solvent ofthe electrolytic solution will be described. The solvent coordinates to(or solvates) the first cations 681. When the first cations 681 servingas main carrier ions move to the vicinity of the surface of the negativeelectrode to form an electric double layer, the solvent, which isneutral and thus is solvated, might move to the vicinity of the surfaceof the negative electrode, resulting in inhibition of formation of theelectric double layer by the first cations 681.

Meanwhile, in the case where an ionic liquid is used as the solvent ofthe electrolytic solution, the anions 683 appear to coordinate to thefirst cations 681. When the first cations 681 move to the vicinity ofthe surface of the negative electrode to form an electric double layer,the anions 683, which assume negative charge, receive an electric fieldin the direction in which the distance from the surface of the negativeelectrode increases. Thus, presumably, the coordination strength of theanions 683 decreases and the anions 683 become detached as the firstcations 681 get closer to the surface of the negative electrode. Thisimplies that the first cations 681 form an electric double layer easilycompared with the case where an aprotic organic solvent is used as asolvent of an electrolytic solution.

[Diffusion Rate of Cations]

In the electrolytic solution of the storage battery of one embodiment ofthe present invention, the diffusion rate of the first cations 681 ispreferably higher than that of the cations 682. The diffusion rate ofcations depends on molecular weight, uneven charge distribution, athree-dimensional structure, and the like.

For example, a large molecular weight of the cations 682 decreases thediffusion rate of the cations 682 but increases the viscosity of theelectrolytic solution. As the viscosity of the electrolytic solutionincreases, the diffusion rate of the first cations 681 serving as maincarrier ions decreases. As the diffusion rate of the first cations 681decreases, the output characteristics of the storage battery aredegraded.

Examples of the cations 682 include aromatic cations and aliphatic oniumcations. Examples of aromatic cations include pyridinium cations andimidazolium cations. Examples of aliphatic onium cations includequaternary ammonium cations, tertiary sulfonium cations, and quaternaryphosphonium cations.

As an ionic liquid containing imidazolium cations, an ionic liquidrepresented by General Formula (G1) below can be used, for example. InGeneral Formula (G1), R¹ represents an alkyl group having 1 to 4 carbonatoms, R² to R⁴ individually represent a hydrogen atom or an alkyl grouphaving 1 to 4 carbon atoms, and R⁵ represents an alkyl group or a mainchain composed of atoms of two or more of C, O, Si, N, S, and P. Themain chain represented by R⁵ may have a substituent. Examples of thesubstituent include an alkyl group and an alkoxy group.

As an ionic liquid containing tertiary sulfonium cations, an ionicliquid represented by General Formula (G2) below can be used, forexample. In General Formula (G2), R²⁵ to R²⁷ individually represent ahydrogen atom, an alkyl group having 1 to 4 carbon atoms, or a phenylgroup. Alternatively, R²⁵ to R²⁷ may individually represent a main chaincomposed of atoms of two or more of C, O, Si, N, S, and P.

As an ionic liquid containing quaternary ammonium cations, an ionicliquid represented by General Formula (G4), (G5), or (G6) below can beused, for example.

In General Formula (G4), R¹² to R¹⁷ individually represent an alkylgroup, a methoxy group, a methoxymethyl group, or a methoxyethyl groupeach having 1 or more and 20 or less carbon atoms, or a hydrogen atom.

In General Formula (G5), R¹⁸ to R²⁴ individually represent an alkylgroup, a methoxy group, a methoxymethyl group, a methoxyethyl group eachhaving 1 or more and 20 or less carbon atoms, or a hydrogen atom.

In General Formula (G6), n and m are greater than or equal to 1 and lessthan or equal to 3. Assume that a is greater than or equal to 0 and lessthan or equal to 6. When n is 1, a is greater than or equal to 0 andless than or equal to 4. When n is 2, a is greater than or equal to 0and less than or equal to 5. When n is 3, a is greater than or equal to0 and less than or equal to 6. Assume that β is greater than or equal to0 and less than or equal to 6. When m is 1, β is greater than or equalto 0 and less than or equal to 4. When m is 2, β is greater than orequal to 0 and less than or equal to 5. When m is 3, β is greater thanor equal to 0 and less than or equal to 6. When a or β is 0, at leastone of two aliphatic rings is unsubstituted. Note that the case whereboth a and β are 0 is excluded. X or Y is a substituent such as astraight-chain or side-chain alkyl group having 1 to 4 carbon atoms, astraight-chain or side-chain alkoxy group having 1 to 4 carbon atoms, ora straight-chain or side-chain alkoxyalkyl group having 1 to 4 carbonatoms.

As an ionic liquid containing pyridinium cations, an ionic liquidrepresented by General Formula (G3) below may be used, for example. InGeneral Formula (G3), R⁶ represents an alkyl group or a main chaincomposed of atoms of two or more of C, O, Si, N, S, and P atoms, and R₇to R₁₁ individually represent a hydrogen atom or an alkyl group having 1to 4 carbon atoms. The main chain represented by R⁶ may have asubstituent. Examples of the substituent include an alkyl group and analkoxy group.

Anions that can be used as the anions 683 can be referred to for A⁻shown in General Formulas (G1) to (G6).

When pyridinium cations with a molecular weight of 150 or less are usedas the cations 682, the cations 682 diffuse in the electrolytic solutionfast and might reach the surface of the negative electrode, easilycausing a decomposition reaction. In contrast, tertiary sulfoniumcations with a molecular weight of 110 or more, imidazolium cations witha molecular weight of 100 or more, and quaternary ammonium cations witha molecular weight of 130 or more are preferably used as the cations682, in which case the diffusion rate of the cations 682 may be lowerthan that of the first cations 681. Note that tertiary sulfonium cationswith a molecular weight of 220 or less, and imidazolium cations with amolecular weight of 250 or less, preferably 175 or less, are morepreferably used as the cations 682, in which case an increase in theviscosity of the electrolytic solution can be small.

The ionic liquid of one embodiment of the present invention ispreferably any of the ionic liquids represented by General Formulas (G1)to (G6), more preferably the ionic liquid represented by General Formula(G1) or (G2), still more preferably the ionic liquid represented byGeneral Formula (G1).

As the anions 683, monovalent amide-based anions, monovalentmethide-based anions, fluorosulfonate anions, perfluoroalkylsulfonateanions, tetrafluoroborate anions, perfluoroalkylborate anions,hexafluorophosphate anions (PF₆ ⁻), perfluoroalkylphosphate anions, orthe like can be used.

As the anions 683, for example, monovalent amide-based anions,monovalent methide-based anions, fluorosulfonate anions (SO₃F⁻),fluoroalkylsulfonate anions, and the like are preferably used.

An example of monovalent amide-based anions is (C_(n)F_(2n+1)SO₂)₂N⁻(n=0 to 3). An example of monovalent cyclic amide-based anions is(CF₂SO₂)₂N⁻. An example of monovalent methide-based anions is(C_(n)F_(2n+1)SO₂)₃C⁻ (n=0 to 3). An example of monovalent cyclicmethide-based anions is (CF₂SO₂)₂C⁻ (CF₃SO₂). An example of fluoroalkylsulfonic acid anions is (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4). An example offluoroalkylborate anions is {BF_(n)(C_(m)H_(k)F_(2m+1−k))_(4−n)}⁻ (n=0to 3, m=1 to 4, and k=0 to 2 m). An example of fluoroalkylphosphateanions is {PF_(n)(C_(m)H_(k)F_(2m+1−k))_(6−n)}⁻ (n=0 to 5, m=1 to 4, andk=0 to 2 m). These anions are preferably used because interactions withthe first cations 681 and the cations 682 may be small. The smallinteraction between the first cations 681 and the anions 683 mayincrease the diffusion rate of the first cations 681. Furthermore, thesmall interaction between the cations 682 and the anions 683 may lowerthe melting point of the ionic liquid in the electrolytic solution.

Examples of monovalent amide-based anions includebis(fluorosulfonyl)amide anions and bis(trifluoromethanesulfonyl)amideanions.

As the anions 683, tetrafluoroborate anions (BF₄ ⁻), fluoroalkylborateanions, or fluoroalkylphosphate anions may alternatively be used.

A compound used for the power storage device of one embodiment of thepresent invention includes a cation represented by General Formula (G7)and its counter anion.

In General Formula (G7), R¹ represents an alkyl group having 1 to 4carbon atoms, R² to R⁴ individually represent a hydrogen atom or analkyl group having 1 to 4 carbon atoms, A¹ to A⁴ individually representa methylene group or an oxygen atom, and at least one of A¹ to A⁴represents an oxygen atom.

When substituents (including substituents represented by A¹ to A⁴ inGeneral Formula (G7)) are bonded to nitrogen of the imidazolium cation,the cation in the ionic liquid has a sterically bulky structure and thusside reactions (e.g., cation insertion into graphite and decompositionof the cation during charge, and gas generation associated with theinsertion and the decomposition) in a battery can be inhibited. However,the viscosity of the ionic liquid is likely to be enhanced as the numberof carbon atoms in A¹ to A⁴ is larger; therefore, it is preferable tocontrol the ionic liquid in accordance with desirable charge anddischarge efficiency and desirable viscosity.

The anions contained in the ionic liquid are monovalent anions whichform the ionic liquid with the imidazolium cations. Examples of theanions include monovalent amide-based anions, monovalent methide-basedanions, fluorosulfonic acid anions (SO₃F⁻), fluoroalkyl sulfonic acidanions, tetrafluoroborate anions (BF₄ ⁻), fluoroalkylborate anions,hexafluorophosphate anions (PF₆ ⁻), and fluoroalkylphosphate anions. Anexample of monovalent amide-based anions is (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0to 3). An example of monovalent cyclic amide-based anions is(CF₂SO₂)₂N⁻. An example of monovalent methide-based anions is(C_(n)F_(2n+1)SO₂)₃C⁻ (n=0 to 3). An example of monovalent cyclicmethide-based anions is (CF₂SO₂)₂C⁻ (CF₃SO₂). An example of fluoroalkylsulfonic acid anions is (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4). An example offluoroalkylborate anions is {BF_(n)(C_(m)H_(k)F_(2m+1−k))_(4−n)}⁻ (n=0to 3, m=1 to 4, and k=0 to 2 m). An example of fluoroalkylphosphateanions is {PF_(n)(C_(m)H_(k)F_(2m+1−k))_(6−n)}⁻ (n=0 to 5, m=1 to 4, andk=0 to 2 m). Note that the anions are not limited thereto.

The anions in the ionic liquid are preferably bis(fluorosulfonyl)amideanions, which are monovalent amide-based anions. An ionic liquidcontaining bis(fluorosulfonyl)amide anions and cations has highconductivity and relatively low viscosity. A power storage deviceincluding the ionic liquid and using graphite for a negative electrodecan be charged and discharged.

A compound used for the power storage device of one embodiment of thepresent invention includes an anion and a cation represented by GeneralFormula (G8).

In General Formula (G8), R¹ represents an alkyl group having 1 to 4carbon atoms, R² to R⁴ individually represent a hydrogen atom or analkyl group having 1 to 4 carbon atoms.

The anions contained in the ionic liquid are monovalent anions whichform the ionic liquid with the imidazolium cations. Examples of theanions include monovalent amide-based anions, monovalent methide-basedanions, fluorosulfonic acid anions (SO₃F⁻), fluoroalkyl sulfonic acidanions, tetrafluoroborate anions (BF₄ ⁻), fluoroalkylborate anions,hexafluorophosphate anions (PF₆ ⁻), and fluoroalkylphosphate anions. Anexample of monovalent amide-based anions is (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0to 3). An example of monovalent cyclic amide-based anions is(CF₂SO₂)₂N⁻. An example of monovalent methide-based anions is(C_(n)F_(2n+1)SO₂)₃C⁻ (n=0 to 3). An example of monovalent cyclicmethide-based anions is (CF₂SO₂)₂C⁻ (CF₃SO₂). An example of fluoroalkylsulfonic acid anions is (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4). An example offluoroalkylborate anions is {BF_(n)(C_(m)H_(k)F_(2m+1−k))_(4−n)}⁻ (n=0to 3, m=1 to 4, and k=0 to 2 m). An example of fluoroalkylphosphateanions is {PF_(n)(C_(m)H_(k)F_(2m+1−k))_(6−n)}⁻ (n=0 to 5, m=1 to 4, andk=0 to 2 m). Note that the anions are not limited thereto.

The anions in the ionic liquid are preferably monovalent amide-basedanions.

A compound used for the power storage device of one embodiment of thepresent invention includes a cation represented by General Formula (G9)and its counter anion.

In General Formula (G9), R¹ represents an alkyl group having 1 to 4carbon atoms, R² to R⁴ individually represent a hydrogen atom or analkyl group having 1 to 4 carbon atoms.

The anions contained in the ionic liquid are monovalent anions whichform the ionic liquid with the imidazolium cations. Examples of theanions include monovalent amide-based anions, monovalent methide-basedanions, fluorosulfonic acid anions (SO₃F⁻), fluoroalkyl sulfonic acidanions, tetrafluoroborate anions (BF₄ ⁻), fluoroalkylborate anions,hexafluorophosphate anions (PF₆ ⁻), and fluoroalkylphosphate anions. Anexample of monovalent amide-based anions is (C_(n)F_(2n+1)SO₂)₂N⁻ (n=0to 3). An example of monovalent cyclic amide-based anions is(CF₂SO₂)₂N⁻. An example of monovalent methide-based anions is(C_(n)F_(2n+1)SO₂)₃C⁻ (n=0 to 3). An example of monovalent cyclicmethide-based anions is (CF₂SO₂)₂C⁻ (CF₃SO₂). An example of fluoroalkylsulfonic acid anions is (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4). An example offluoroalkylborate anions is {BF_(n)(C_(m)H_(k)F_(2m+1−k))_(4−n)}⁻ (n=0to 3, m=1 to 4, and k=0 to 2 m). An example of fluoroalkylphosphateanions is {PF_(n)(C_(m)H_(k)F_(2m+1−k))_(6−n)}⁻ (n=0 to 5, m=1 to 4, andk=0 to 2 m). Note that the anions are not limited thereto.

The anions in the ionic liquid are preferably monovalent amide-basedanions.

The alkyl groups in the ionic liquids represented by General Formulas(G7) to (G9) may each be either a straight-chain alkyl group or abranched-chain alkyl group, such as an ethyl group or a tert-butylgroup. In the ionic liquid represented by General Formula (G7), it ispreferred that A¹ to A⁴ not have an oxygen-oxygen bond (peroxide). Anoxygen-oxygen single bond breaks very easily and is highly reactive;thus, the ionic liquid with the bond might be explosive. Thus, the ionicliquid is not suitable for power storage devices.

Specific examples of the cation represented by General Formula (G1)below include Structural Formulas (111) to (174).

Specific examples of the cation represented by General Formula (G2)below include Structural Formulas (201) to (215).

[Negative Electrode Active Material]

In the case where the active material is a negative electrode activematerial, for example, an alloy-based material, a carbon-based material,or the like can be used.

For the negative electrode active material, an element which enablescharge-discharge reactions by an alloying reaction and a dealloyingreaction with lithium can be used. For example, a material containing atleast one of silicon, tin, gallium, aluminum, germanium, lead, antimony,bismuth, silver, zinc, cadmium, indium, and the like can be used. Suchelements have higher capacity than carbon. In particular, silicon has asignificantly high theoretical capacity of 4200 mAh/g. For this reason,silicon is preferably used as the negative electrode active material.Alternatively, a compound containing any of the above elements may beused. Examples of the compound include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like. Here, anelement that enables charge-discharge reactions by an alloying reactionand a dealloying reaction with lithium, a compound containing theelement, and the like may be referred to as an alloy-based material.

It is known that the volume of an alloy-based material of silicon or thelike expands due to an alloying reaction with lithium as disclosed inPatent Document 1. The expanded volume contracts due to a dealloyingreaction with lithium. Charge and discharge of the storage battery areaccompanied by expansion and contraction of the negative electrodeactive material.

In the state where the negative electrode active material expands, thecoating film 685 is formed on the surface of the negative electrodeactive material. After that, the negative electrode active materialwhose surface is provided with the coating film 685 contracts. In thecontraction, stress from the coating film 685 is imposed on the negativeelectrode active material, easily forming a crack or the like in thenegative electrode active material, for example. Furthermore, in thecontraction, the coating film 685 might be caught in the crack,expanding the crack or causing pulverization of the active material. Amixed region of the pulverized negative electrode active material andthe coating film caught in the crack formed in the negative electrodeactive material is formed on the surface of the negative electrodeactive material. The pulverized negative electrode active material mightlose electrical conduction. In that case, no charge and dischargereactions might occur, resulting in a decrease in the capacity of thestorage battery. Thus, the negative electrode active material ispreferably prevented from being cracked and pulverized.

In this specification and the like, SiO refers, for example, to siliconmonoxide. SiO can alternatively be expressed as SiOx. Here, x preferablyhas an approximate value of 1. For example, x is preferably 0.2 or moreand 1.5 or less, more preferably 0.3 or more and 1.2 or less.

As the carbon-based material, graphite, graphitizing carbon (softcarbon), non-graphitizing carbon (hard carbon), a carbon nanotube,graphene, carbon black, or the like can be used.

Examples of graphite include artificial graphite and natural graphite.Examples of artificial graphite include meso-carbon microbeads (MCMB),coke-based artificial graphite, and pitch-based artificial graphite. Asartificial graphite, spherical graphite having a spherical shape can beused. For example, MCMB is preferably used because it may have aspherical shape. Moreover, MCMB may preferably be used because it canrelatively easily have a small surface area. Examples of naturalgraphite include flake graphite and spherical natural graphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.1 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into the graphite (whilea lithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferred because of its advantages such as arelatively high capacity per unit volume, relatively small volumeexpansion, low cost, and higher level of safety than that of a lithiummetal.

Note that it is known that when lithium ions are intercalated ingraphite, the interlayer distance of graphite increases from 0.336 nm to0.370 nm, for example (see Non-patent Document 1, pp. 333-334). That is,the interlayer distance increases by approximately 11%.

Alternatively, for the negative electrode active materials, an oxidesuch as titanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active materials,Li_(3−x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive materials and thus the negative electrode active materials can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈. Inthe case of using a material containing lithium ions as a positiveelectrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedfor the negative electrode active materials; for example, a transitionmetal oxide which does not form an alloy with lithium, such as cobaltoxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used.Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

[Predoping]

In the case where a coating film is formed in the initial charge anddischarge cycle, an irreversible reaction occurs. For example, in thecase where one of an irreversible reaction at the positive electrode andan irreversible reaction at the negative electrode is greater, thebalance between charge and discharge might be disrupted, resulting in adecrease in the capacity of the storage battery. Replacing an electrodeused as a counter electrode after charge and discharge using the counterelectrode are performed can inhibit a decrease in capacity. For example,charge or charge and discharge are performed using a positive electrodein combination with a negative electrode, and then, the positiveelectrode is removed to be replaced with another positive electrode inthe storage battery. This may inhibit a decrease in the capacity of thestorage battery. This method may be called predoping or preaging.

Inhibiting deposition of the coating film 685 on the surface of thenegative electrode active material can prevent the negative electrodeactive material from being cracked and pulverized in some cases.

The negative electrode of one embodiment of the present inventionpreferably includes a first element and carbon. The first element ispreferably any of silicon, tin, gallium, aluminum, germanium, lead,antimony, bismuth, silver, zinc, cadmium, and indium.

[Layers Included in Negative Electrode]

The negative electrode of one embodiment of the present inventionincludes a negative electrode active material and a first layer on thesurface of the negative electrode active material. The first layer maybe called a coating film. The thickness of the first layer is preferablylarger than or equal to 10 nm and smaller than or equal to 1000 nm, morepreferably larger than or equal to 50 nm and smaller than or equal to200 nm, still more preferably larger than or equal to 50 nm and smallerthan or equal to 100 nm.

Alternatively, the negative electrode of one embodiment of the presentinvention includes a first region, a second region in contact with asurface of the first region, and a third region in contact with asurface of the second region. The second region and the third regioneach have a shape of a layer. The thickness of the second region ispreferably larger than or equal to 10 nm and smaller than or equal to500 nm, more preferably larger than or equal to 50 nm and smaller thanor equal to 200 nm, still more preferably larger than or equal to 50 nmand smaller than or equal to 100 nm. The thickness of the third regionis preferably larger than or equal to 10 nm and smaller than or equal to1000 nm, more preferably larger than or equal to 50 nm and smaller thanor equal to 200 nm, still more preferably larger than or equal to 50 nmand smaller than or equal to 100 nm. The atomic ratio of carbon to thefirst element in the first region is x₁:y₁. The atomic ratio of carbonto the first element in the second region is x₂:y₂. The atomic ratio ofcarbon to the first element in the third region is x₃:y₃. x₁/y₁ ispreferably smaller than or equal to 3, more preferably smaller than orequal to 1.5. x₂/y₂ is preferably larger than or equal to 0.1 andsmaller than 10, more preferably larger than or equal to 0.3 and smallerthan or equal to 5. x₃/y₃ is preferably larger than or equal to 5, morepreferably larger than or equal to 10, still more preferably larger thanor equal to 20.

When the intensity ratio of carbon to the first element in the firstregion, which is obtained by EDX analysis, is x₁:y₁, the intensity ratioof carbon to the first element in the second region is x₂:y₂, and theintensity ratio of carbon to the first element in the third region isx₃:y₃, x₁/y₁ is preferably smaller than or equal to 0.3, x₂/y₂ ispreferably larger than or equal to 0.1 and smaller than or equal to 5,more preferably larger than or equal to 0.3 and smaller than or equal to3, and x₃/y₃ is preferably larger than or equal to 2, more preferablylarger than or equal to 5, still more preferably larger than or equal to10.

When the intensity ratio of carbon to the first element in the firstregion, which is obtained by EELS analysis, is x₁:y₁, the intensityratio of carbon to the first element in the second region is x₂:y₂, andthe intensity ratio of carbon to the first element in the third regionis x₃:y₃, x₁/y₁ is preferably smaller than or equal to 0.3, x₂/y₂ ispreferably larger than or equal to 0.1 and smaller than 5, morepreferably larger than or equal to 0.3 and smaller than or equal to 3,and x₃/y₃ is preferably larger than or equal to 2, more preferablylarger than or equal to 5, still more preferably larger than or equal to10.

FIG. 54A illustrates an example where a negative electrode includes afirst region 551, a second region 552, and a third region 553. Asillustrated in FIG. 54B, the second region 552 may be a mixed region ofa material of the first region 551 and a material of the third region553. For example, the first region 551 is a negative electrode activematerial, and the third region 553 is a coating film formed bydeposition of a decomposition product of an electrolytic solution. Inthe case where the negative electrode active material includes the firstelement, e.g., silicon, and the decomposition product of theelectrolytic solution includes carbon, the first region 551 includessilicon and carbon such that the amount of carbon is smaller than thatof silicon. The third region 553 includes silicon and carbon such thatthe amount of carbon is larger than that of silicon. That is, x₃/y₃ islarger than x₁/y₁.

For the S2p spectrum of XPS analysis of the negative electrode of oneembodiment of the present invention, the spectral intensity at around168 eV is two or more times, three or more times, or four or more timesthe spectral intensity at around 163 eV, for example.

A current collector included in each of the positive electrode and thenegative electrode can be formed using a material that has highconductivity, such as a metal of stainless steel, gold, platinum,aluminum, titanium, or the like, or an alloy thereof. In the case wherethe current collector is used in the positive electrode, it is preferredthat it not dissolve at the potential of the positive electrode. In thecase where the current collector is used in the negative electrode, itis preferred that it not be alloyed with carrier ions such as lithiumions. Alternatively, the current collector can be formed using analuminum alloy to which an element that improves heat resistance, suchas silicon, titanium, neodymium, scandium, or molybdenum, is added.Still alternatively, a metal element that forms silicide by reactingwith silicon can be used. Examples of the metal element that formssilicide by reacting with silicon include zirconium, titanium, hafnium,vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt,nickel, and the like. The current collector can have any of variousshapes including a foil-like shape, a plate-like shape (sheet-likeshape), a net-like shape, a punching-metal shape, and an expanded-metalshape. The current collector preferably has a thickness of 5 μm to 30 μminclusive.

Examples of a positive electrode active material include a compositeoxide with an olivine crystal structure, a composite oxide with alayered rock-salt crystal structure, and a composite oxide with a spinelcrystal structure.

As the positive electrode active material, a compound such as LiFeO₂,LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂ can be used. LiCoO₂ isparticularly preferable because it has a high capacity, stability in theair higher than that of LiNiO₂, and thermal stability higher than thatof LiNiO₂, for example. It is preferable to add a small amount oflithium nickel oxide (LiNiO₂ or LiNi_(1−x)M_(x)O₂ (M=Co, Al, or thelike)) to a lithium-containing material with a spinel crystal structurewhich contains manganese such as LiMn₂O₄ because the characteristics ofa secondary battery using such a material can be improved.

The average diameter of primary particles of the positive electrodeactive material is preferably greater than or equal to 5 nm and lessthan or equal to 50 μm, more preferably greater than or equal to 100 nmand less than or equal to 500 nm, for example. Furthermore, the specificsurface area is preferably greater than or equal to 5 m²/g and less thanor equal to 15 m²/g. Furthermore, the average diameter of secondaryparticles is preferably greater than or equal to 5 μm and less than orequal to 50 μm. Note that the average particle sizes can be measuredwith a particle size distribution analyzer or the like using a laserdiffraction and scattering method or by observation with a scanningelectron microscope (SEM) or a transmission electron microscope (TEM).The specific surface area can be measured by a gas adsorption method.

Another example of the positive electrode active material is alithium-manganese composite oxide that is represented by a compositionformula Li_(a)Mn_(b)M_(c)O_(d). Here, the element M is preferably ametal element other than lithium and manganese, or silicon orphosphorus, more preferably nickel. Furthermore, in the case where thewhole particle of a lithium-manganese composite oxide is measured, it ispreferable to satisfy the following at the time of discharging:0<a/(b+c)<2; c>0; and 0.26 (b+c)/d<0.5. To achieve a high capacity, thelithium-manganese composite oxide preferably includes a region where thesurface portion and the middle portion are different in the crystalstructure, the crystal orientation, or the oxygen content. In order thatsuch a lithium-manganese composite oxide can be obtained, thecomposition formula is preferably Li_(a)Mn_(b)Ni_(c)O_(d) satisfying thefollowing: 1.6≤a≤1.848; 0.19≤c/b≤0.935; and 2.5≤d≤3. Furthermore, it isparticularly preferable to use a lithium-manganese composite oxiderepresented by a composition formula Li_(1.68)Mno_(0.8062)Ni_(0.318)O₃.In this specification and the like, a lithium-manganese composite oxiderepresented by a composition formula Li_(1.68)Mno_(0.8062)Ni_(0.318)O₃refers to that formed at a ratio (molar ratio) of the amounts of rawmaterials of Li₂CO₃:MnCO₃:NiO=0.84:0.8062:0.318. Although thislithium-manganese composite oxide is represented by a compositionformula Li_(1.68)Mn_(0.8062)Ni_(0.318)O₃, the composition might deviatefrom this.

Note that the ratios of metal, silicon, phosphorus, and other elementsto the total composition in the whole particle of a lithium-manganesecomposite oxide can be measured with, for example, an inductivelycoupled plasma mass spectrometer (ICP-MS). The ratio of oxygen to thetotal composition in the whole particle of a lithium-manganese compositeoxide can be measured by, for example, energy dispersive X-rayspectroscopy (EDX). Alternatively, the ratio of oxygen to the totalcomposition in the whole particle of a lithium-manganese composite oxidecan be measured by ICP-MS combined with fusion gas analysis and valenceevaluation of X-ray absorption fine structure (XAFS) analysis. Note thatthe lithium-manganese composite oxide is an oxide containing at leastlithium and manganese, and may contain at least one selected fromchromium, cobalt, aluminum, nickel, iron, magnesium, molybdenum, zinc,indium, gallium, copper, titanium, niobium, silicon, phosphorus, and thelike.

FIGS. 2A and 2B each illustrate an example of a cross-sectional view ofa particle of a lithium-manganese composite oxide having a region withdifferent crystal structures, crystal orientations, or oxygen contents.

As illustrated in FIG. 2A, the lithium-manganese composite oxide havinga region with different crystal structures, crystal orientations, oroxygen contents preferably include a region 331, a region 332, and aregion 333. The region 332 is in contact with at least part of the outerside of the region 331. Here, the term “outer side” refers to the sidecloser to a surface of a particle. The region 333 preferably includes aregion corresponding to a surface of a particle containing thelithium-manganese composite oxide.

As shown in FIG. 2B, the region 331 may include a region not coveredwith the region 332. The region 332 may include a region not coveredwith the region 333. Furthermore, the region 331 may include a region incontact with the region 333, for example. Furthermore, the region 331may include a region covered with neither the region 332 nor the region333.

The region 332 preferably has composition different from that of theregion 331.

For example, the case will be described where the composition of theregion 331 and that of the region 332 are separately measured and theregion 331 and the region 332 each contain lithium, manganese, theelement M, and oxygen; the atomic ratio of lithium to manganese, theelement M, and oxygen in the region 331 is represented by a1:b1:c1:d1;and the atomic ratio of lithium to manganese, the element M, and oxygenin the region 332 is represented by a2:b2:c2:d2. Note that thecomposition of each of the region 331 and the region 332 can be measuredby, for example, EDX using a TEM. In measurement by EDX, the proportionof lithium is sometimes difficult to measure. Thus, a difference betweenthe region 331 and the region 332 in composition except for lithium willbe described below. Here, d1/(b1+c1) is preferably greater than or equalto 2.2, more preferably greater than or equal to 2.3, still morepreferably greater than or equal to 2.35 and less than or equal to 3.Furthermore, d2/(b2+c2) is preferably less than 2.2, more preferablyless than 2.1, much more preferably greater than or equal to 1.1 andless than or equal to 1.9. In this case, the composition of the wholeparticle of lithium-manganese composite oxide including the region 331and the region 332 also preferably satisfies the above inequality:0.26≤(b+c)/d<0.5.

The valence of manganese in the region 332 may be different from that ofmanganese in the region 331. The valence of the element Min the region332 may be different from that of the element Min the region 331.

Specifically, the region 331 is preferably a lithium-manganese compositeoxide having a layered rock-salt crystal structure. The region 332 ispreferably a lithium-manganese composite oxide having a spinel crystalstructure.

Here, in the case where the compositions of the regions or valences ofelements in the regions are spatially distributed, the compositions orvalences in a plurality of portions are obtained, the average valuesthereof are calculated, and the average values are regarded as thecompositions or valences of the regions, for example.

A transition layer may be provided between the region 332 and the region331. The transition layer is a region where the composition, crystalstructure, or crystal lattice constant changes continuously or stepwise.A mixed layer may be provided between the region 332 and the region 331.The mixed layer is a layer in which, for example, two or more crystalshaving different crystal orientations are mixed, two or more crystalshaving different crystal structures are mixed, or two or more crystalshaving different compositions are mixed.

The region 333 preferably contains carbon or a metal compound. Examplesof the metal include cobalt, aluminum, nickel, iron, manganese,titanium, zinc, and lithium. Examples of the metal compound include anoxide and a fluoride of the metal.

It is particularly preferable that the region 333 contain carbon. Sincecarbon has high conductivity, the particle covered with carbon in theelectrode of the power storage device can reduce the resistance of theelectrode, for example. The region 333 preferably includes a graphenecompound. The use of the graphene compound in the region 333 allows thelithium-manganese composite oxide particle to be efficiently coated withthe region 333. The graphene compound will be described later. Theregion 333 may include, specifically, graphene or graphene oxide, forexample. Furthermore, graphene formed by reducing graphene oxide ispreferably used as graphene. Graphene has excellent electricalcharacteristics of high conductivity and excellent physical propertiesof high flexibility and high mechanical strength. When graphene oxide isused for the region 333 and is reduced, the region 332 in contact withthe region 333 is oxidized in some cases.

When the region 333 includes a graphene compound, the secondary batteryusing the lithium-manganese composite oxide as a positive electrodematerial can have improved cycle performance.

The thickness of the region 333 is preferably greater than or equal to0.4 nm and less than or equal to 40 nm.

Furthermore, the average diameter of primary particles of thelithium-manganese composite oxide is preferably greater than or equal to5 nm and less than or equal to 50 mm, more preferably greater than orequal to 100 nm and less than or equal to 500 nm, for example.Furthermore, the specific surface area is preferably greater than orequal to 5 m²/g and less than or equal to 15 m²/g. Furthermore, theaverage diameter of secondary particles is preferably greater than orequal to 5 μm and less than or equal to 50 μm.

The positive electrode and the negative electrode may include aconductive additive. Examples of the conductive additive include acarbon material, a metal material, and a conductive ceramic material.Alternatively, a fiber material may be used as the conductive additive.The content of the conductive additive in the active material layer ispreferably greater than or equal to 1 wt % and less than or equal to 10wt %, more preferably greater than or equal to 1 wt % and less than orequal to 5 wt %.

A network for electric conduction can be formed in the electrode by theconductive additive. The conductive additive also allows maintaining ofa path for electric conduction between the positive electrode activematerial particles. The addition of the conductive additive to theactive material layer increases the electric conductivity of the activematerial layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. Examples ofcarbon fiber include mesophase pitch-based carbon fiber, isotropicpitch-based carbon fiber, carbon nanofiber, and carbon nanotube. Carbonnanotube can be formed by, for example, a vapor deposition method. Otherexamples of the conductive additive include carbon materials such ascarbon black (e.g., acetylene black (AB)), graphite (black lead)particles, graphene, and fullerene. Alternatively, metal powder or metalfibers of copper, nickel, aluminum, silver, gold, or the like, aconductive ceramic material, or the like can be used.

Alternatively, a graphene compound may be used as the conductiveadditive.

A graphene compound may have excellent electrical characteristics ofhigh conductivity and excellent physical properties of high flexibilityand high mechanical strength. A graphene compound has a planar shape andenables low-resistant surface contact. Furthermore, a graphene compoundhas extremely high conductivity even with a small thickness in somecases and thus allows a conductive path to be formed in an activematerial layer efficiently even with a small amount. Thus, a graphenecompound is preferably used as a conductive additive, in which case thearea where an active material and the conductive additive are in contactwith each other can be increased and electrical resistance may bereduced. Here, it is particularly preferred that graphene, multilayergraphene, or reduced graphene oxide (hereinafter referred to as RGO) beused as a graphene compound. Note that RGO refers to a compound obtainedby reducing graphene oxide (GO), for example.

In the case where an active material with a small particle diameter(e.g., 1 μm or less) is used, the specific surface area of the activematerial is large and thus more conductive paths for the active materialparticles are needed. In such a case, it is particularly preferred thata graphene compound with extremely high conductivity that canefficiently form a conductive path even in a small amount is used.

A cross-sectional structure example of the active material layercontaining a graphene compound as a conductive additive will bedescribed below.

FIG. 3A is a longitudinal sectional view of the active material layer.The active material layer includes the active material particles 103,graphene compounds 321 as a conductive additive, and a binder (notillustrated). Here, graphene or multilayer graphene can be used as thegraphene compound 321, for example. The graphene compound 321 preferablyhas a sheet-like shape. The graphene compound 321 may have a sheet-likeshape formed of a plurality of sheets of multilayer graphene and/or aplurality of sheets of graphene that partly overlap with each other.

The longitudinal section of the active material layer in FIG. 3A showssubstantially uniform dispersion of the graphene compounds 321 in theactive material layer. The graphene compounds 321 are schematicallyshown by thick lines in FIG. 3A but are actually thin films each havinga thickness corresponding to the thickness of a single layer or amulti-layer of carbon molecules. The plurality of graphene compounds 321are formed in such a way as to wrap, coat, or adhere to the surfaces ofthe plurality of active material particles 103, so that the graphenecompounds 321 make surface contact with the active material particles103. Furthermore, the graphene compounds 321 are also in surface contactwith each other; consequently, the plurality of graphene compounds 321form a three-dimensional network for electric conduction.

Here, a plurality of graphene compounds are bonded to each other to formnet-like graphene compound sheet (hereinafter referred to as a graphenecompound net or a graphene net). The graphene net covering the activematerial can function as a binder for binding active materials. Theamount of the binder can thus be reduced, or the binder does not have tobe used. This can increase the proportion of the active material in theelectrode volume or weight. That is to say, the capacity of the powerstorage device can be increased.

Here, it is preferable to perform reduction after a layer to be theactive material layer is formed in such a manner that graphene oxideflakes are used as the graphene compounds 321 and mixed with an activematerial. When graphene oxide flakes with extremely high dispersibilityin a polar solvent are used for the formation of the graphene compounds321, the graphene compounds 321 can be substantially uniformly dispersedin the active material layer. The solvent is removed by volatilizationfrom a dispersion medium in which graphene oxide flakes are uniformlydispersed, and the graphene oxide is reduced; hence, the graphenecompounds 321 remaining in the active material layer partly overlap witheach other and are dispersed such that surface contact is made, therebyforming a three-dimensional conduction path. Note that graphene oxideflakes can be reduced either by heat treatment or with the use of areducing agent, for example.

Unlike a conductive additive in the form of particles, such as acetyleneblack, which makes point contact with an active material, the graphenecompound 321 is capable of making low-resistance surface contact;accordingly, the electrical conduction between the active materialparticles 103 and the graphene compounds 321 can be improved with asmaller amount of the graphene compounds 321 than that of a normalconductive additive. Thus, the proportion of the active materialparticles 103 in the active material layer can be increased.Accordingly, the discharge capacity of a power storage device can beincreased.

The positive electrode and the negative electrode may each include abinder. As the binder, a rubber material such as styrene-butadienerubber (SBR), styrene-isoprene-styrene rubber, acrylonitrile-butadienerubber, butadiene rubber, or ethylene-propylene-diene copolymer can beused. Alternatively, fluororubber can be used as the binder.

For the binder, for example, water-soluble polymers are preferably used.As the water-soluble polymers, a polysaccharide or the like can be used.As the polysaccharide, a cellulose derivative such as carboxymethylcellulose (CMC), methyl cellulose, ethyl cellulose, hydroxypropylcellulose, diacetyl cellulose, or regenerated cellulose, starch, or thelike can be used. It is more preferred that such water-soluble polymersbe used in combination with any of the above rubber materials.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (PMMA), sodiumpolyacrylate, polyvinyl alcohol (PVA), polyethylene oxide (PEO),polypropylene oxide, polyimide, polyvinyl chloride,polytetrafluoroethylene, polyethylene, polypropylene, polyisobutylene,polyethylene terephthalate, nylon, polyvinylidene fluoride (PVdF),polyacrylonitrile (PAN), ethylene-propylene-diene polymer, polyvinylacetate, or nitrocellulose is preferably used.

Two or more of the above materials may be used in combination for thebinder.

For example, a material having a significant viscosity modifying effectand another material may be used in combination. For example, a rubbermaterial or the like has high adhesion or high elasticity but may havedifficulty in viscosity modification when mixed in a solvent. In such acase, a rubber material or the like is preferably mixed with a materialhaving a significant viscosity modifying effect, for example. As amaterial having a significant viscosity modifying effect, for example, awater-soluble polymer is preferably used. An example of a water-solublepolymer having an especially significant viscosity modifying effect isthe above-mentioned polysaccharide; for example, a cellulose derivativesuch as carboxymethyl cellulose (CMC), methyl cellulose, ethylcellulose, hydroxypropyl cellulose, diacetyl cellulose, or regeneratedcellulose, or starch can be used.

Note that a cellulose derivative such as carboxymethyl cellulose obtainsa higher solubility when converted into a salt such as a sodium salt oran ammonium salt of carboxymethyl cellulose, and accordingly, easilyexerts an effect as a viscosity modifier. The high solubility can alsoincrease the dispersibility of an active material and other componentsin the formation of slurry for an electrode. In this specification,cellulose and a cellulose derivative used as a binder of an electrodeinclude salts thereof.

The water-soluble polymers stabilize viscosity by being dissolved inwater and allow stable dispersion of the active material and anothermaterial combined as a binder such as styrene-butadiene rubber in anaqueous solution. Furthermore, a water-soluble polymer is expected to beeasily and stably adsorbed to an active material surface because it hasa functional group. Many cellulose derivatives such as carboxymethylcellulose have functional groups such as a hydroxyl group and a carboxylgroup. Because of functional groups, polymers are expected to interactwith each other and cover an active material surface in a large area.

In the case where the binder covering or being in contact with theactive material surface forms a film, the film is expected to serve as apassivation film to suppress the decomposition of the electrolyticsolution. Here, the passivation film refers to a film without electricconductivity or a film with extremely low electric conductivity, and caninhibit the decomposition of an electrolytic solution at a potential atwhich a battery reaction occurs in the case where the passivation filmis formed on the active material surface, for example. It is preferredthat the passivation film can conduct lithium ions while suppressingelectric conduction.

[Method for Fabricating Electrode]

In examples of methods for fabricating negative and positive electrodes,a slurry is formed and an electrode is fabricated by application of theslurry. A method for forming a slurry used for electrode fabricationwill be described.

A polar solvent is preferably used as the solvent used for formation ofthe slurry. Examples of the polar solvent include water, methanol,ethanol, acetone, tetrahydrofuran (THF), dimethylformamide (DMF),N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and a mixedsolution of any two or more of the above.

First, the active material, the conductive additive, and the binder aremixed to form Mixture A (Step S110). Next, the solvent is added toMixture A and kneading (mixing with a high viscosity) is performed, sothat Mixture B is formed (Step S120). Here, Mixture B is preferably in apaste form, for example. In the case where a second binder is added in alater step S141, a first binder is not necessarily added in Step S110 insome cases.

Next, the solvent is added to Mixture B and kneading is performed, sothat Mixture C is formed (Step S130).

Next, in the case where the second binder is used, the second binder isadded to form Mixture D (Step S141). At this time, a solvent may beadded. In the case where the second binder is not used, a solvent isadded as needed to form Mixture E (Step S142).

Then, Mixture D or Mixture E is mixed in a reduced-pressure atmosphereto form Mixture F (Step S150). At this time, a solvent may be added. Inthe mixing and kneading steps in Steps S110 to S150, a mixer may beused, for example.

Next, the viscosity of Mixture F is measured (Step S160). After that, asolvent is added as needed to adjust the viscosity. Through the abovesteps, slurry for application of the active material layer is obtained.

Here, for example, the higher the viscosity of Mixtures C to F in StepsS130 to S160 is, the higher the dispersibility of the active material,the binder, and the conductive additive in the mixtures is (the betterthey are mixed together), in some cases. Thus, the viscosity F ispreferably higher. However, an excessively high viscosity of Mixture Fis not preferred in terms of productivity because it might reduce theelectrode application speed.

Next, a method for forming the active material layer on the currentcollector with the use of the formed slurry will be described.

First, the slurry is applied to the current collector. Before theapplication of the slurry, surface treatment may be performed on thecurrent collector. Examples of surface treatment include coronadischarge treatment, plasma treatment, and undercoat treatment. Here,the “undercoat” refers to a film formed over a current collector beforeapplication of slurry onto the current collector for the purpose ofreducing the interface resistance between an active material layer andthe current collector or increasing the adhesion between the activematerial layer and the current collector. Note that the undercoat is notnecessarily formed in a film shape, and may be formed in an islandshape. In addition, the undercoat may serve as an active material tohave capacity. For the undercoat, a carbon material can be used, forexample. Examples of the carbon material include graphite, carbon blacksuch as acetylene black and ketjen black (registered trademark), and acarbon nanotube.

For the application of the slurry, a slot die method, a gravure method,a blade method, or combination of any of them can be used. Furthermore,a continuous coater or the like may be used for the application.

Then, the solvent of the slurry is volatilized to form the activematerial layer. The steps for volatilizing the solvent of the slurry areas follows, for example.

The step of volatilizing the solvent of the slurry is preferablyperformed at a temperature in the range from 50° C. to 200° C.inclusive, more preferably from 60° C. to 150° C. inclusive.

Heat treatment is performed using a hot plate at 30° C. or higher and70° C. or lower in an air atmosphere for longer than or equal to 10minutes, and then, for example, another heat treatment is performed atroom temperature or higher and 100° C. or lower in a reduced-pressureenvironment for longer than or equal to 1 hour and shorter than or equalto 10 hours.

Alternatively, heat treatment may be performed using a drying furnace orthe like. In the case of using a drying furnace, the heat treatment isperformed at 30° C. or higher and 120° C. or lower for longer than orequal to 30 seconds and shorter than or equal to 20 minutes, forexample.

The temperature may be increased in stages. For example, after heattreatment is performed at 60° C. or lower for shorter than or equal to10 minutes, another heat treatment may further be performed at higherthan or equal to 65° C. for longer than or equal to 1 minute.

The thickness of the active material layer formed through the abovesteps is, for example, preferably greater than or equal to 5 μm and lessthan or equal to 300 μm, more preferably greater than or equal to 10 μmand less than or equal to 150 μm. Furthermore, the loading of the activematerial in the active material layer is, for example, preferablygreater than or equal to 2 mg/cm² and less than or equal to 50 mg/cm².

The active material layer may be formed over only one surface of thecurrent collector, or the active material layers may be formed such thatthe current collector is sandwiched therebetween. Alternatively, theactive material layers may be formed such that part of the currentcollector is sandwiched therebetween.

After the volatilization of the solvent from the active material layer,pressing may be performed by a compression method such as a roll pressmethod or a flat plate press method. In performing pressing, heat may beapplied.

Note that the active material layer may be predoped. There is noparticular limitation on the method for predoping the active materiallayer. For example, the active material layer may be predopedelectrochemically. For example, before a battery is assembled, theactive material layer can be predoped with lithium in an electrolyticsolution described later with the use of a lithium metal as a counterelectrode. Alternatively, predoping may be performed using a positiveelectrode for predoping as a counter electrode of a negative electrode,and then, the positive electrode for predoping may be removed. Predopingcan particularly inhibit a decrease in initial charge and dischargeefficiency, leading to an increase in the capacity of the storagebattery.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 2

In this embodiment, power storage devices of embodiments of the presentinvention will be described.

Examples of the power storage device of one embodiment of the presentinvention include a secondary battery that utilizes an electrochemicalreaction, such as such as a lithium-ion battery, an electrochemicalcapacitor such as an electric double-layer capacitor or a redoxcapacitor, an air battery, and a fuel battery.

<Thin Storage Battery>

FIG. 4 illustrates a thin storage battery as an example of a powerstorage device. When a flexible thin storage battery is used in anelectronic device at least part of which is flexible, the storagebattery can be bent as the electronic device is bent.

FIG. 4 is an external view of a storage battery 500, which is a thinstorage battery. FIG. 5A is a cross-sectional view along dashed-dottedline A1-A2 in FIG. 4 , and FIG. 5B is a cross-sectional view alongdashed-dotted line B1-B2 in FIG. 4 . The storage battery 500 includes apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolytic solution 508, and an exterior body 509. The separator 507is provided between the positive electrode 503 and the negativeelectrode 506 in the exterior body 509. The electrolytic solution 508 iscontained in the exterior body 509.

The electrolytic solution used for a power storage device is preferablyhighly purified and contains a small amount of dust particles andelements other than the constituent elements of the electrolyticsolution (hereinafter, also simply referred to as impurities).Specifically, the weight ratio of impurities to the electrolyticsolution is less than or equal to 1%, preferably less than or equal to0.1%, and more preferably less than or equal to 0.01%.

Furthermore, an additive agent such as vinylene carbonate, propanesultone (PS), tert-butylbenzene (TBB), fluoroethylene carbonate (FEC),or LiBOB may be added to the electrolytic solution. The concentration ofsuch an additive agent in the whole solvent is, for example, higher thanor equal to 0.1 wt % and lower than or equal to 5 wt %.

Alternatively, a gel polymer electrolyte obtained in such a manner thata polymer is swelled with an electrolytic solution may be used.

Examples of host polymers include a polymer having a polyalkylene oxidestructure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile;and a copolymer containing any of them. For example, PVDF-HFP, which isa copolymer of PVDF and hexafluoropropylene (HFP) can be used. Theformed polymer may be porous.

Instead of the electrolytic solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including amacromolecular material such as a polyethylene oxide (PEO)-basedmacromolecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, the battery can be entirely solidified; therefore, there isno possibility of liquid leakage and thus the safety of the battery isdramatically increased.

As the separator 507, paper; nonwoven fabric; glass fiber; ceramics;synthetic fiber containing nylon (polyamide), vinylon (polyvinylalcohol-based fiber), polyester, acrylic, polyolefin, or polyurethane;or the like can be used.

The separator 507 is preferably formed to have a bag-like shape tosurround one of the positive electrode 503 and the negative electrode506. For example, as illustrated in FIG. 6A, the separator 507 is foldedin two so that the positive electrode 503 is sandwiched, and sealed witha sealing member 514 in a region outside the region overlapping with thepositive electrode 503; thus, the positive electrode 503 can be reliablysupported inside the separator 507. Then, as illustrated in FIG. 6B, thepositive electrodes 503 surrounded by the separators 507 and thenegative electrodes 506 are alternately stacked and provided in theexterior body 509, whereby the storage battery 500 can be formed.

Next, aging after fabrication of a storage battery will be described.Aging is preferably performed after fabrication of a storage battery.The aging can be performed under the following conditions, for example.Charge is performed at a rate of 0.001 C or more and 0.2 C or less at atemperature higher than or equal to room temperature and lower than orequal to 50° C. In the case where the reaction potential of the positiveelectrode or the negative electrode is out of the range of the potentialwindow of the electrolytic solution 508, the electrolytic solution isdecomposed by charge and discharge operations of a storage battery insome cases. In the case where the electrolytic solution is decomposedand a gas is generated and accumulated in the cell, the electrolyticsolution cannot be in contact with a surface of the electrode in someregions. That is to say, an effectual reaction area of the electrode isreduced and effectual resistance is increased.

When the resistance is extremely increased, the negative electrodepotential is lowered. Consequently, lithium is intercalated intographite and lithium is deposited on the surface of graphite. Thelithium deposition might reduce capacity. For example, if a film or thelike is grown on the surface after lithium deposition, lithium depositedon the surface cannot be dissolved again. This lithium cannot contributeto capacity. In addition, when deposited lithium is physically collapsedand conduction with the electrode is lost, the lithium also cannotcontribute to capacity. Therefore, the gas is preferably released beforethe negative electrode potential reaches the potential of lithiumbecause of an increase in a charging voltage.

After the release of the gas, the charging state may be maintained at atemperature higher than room temperature, preferably higher than orequal to 30° C. and lower than or equal to 60° C., more preferablyhigher than or equal to 35° C. and lower than or equal to 50° C. for,for example, 1 hour or more and 100 hours or less. In the initialcharge, an electrolytic solution decomposed on the surface forms a filmon a surface of graphite. The formed coating film may thus be densifiedwhen the charging state is held at a temperature higher than roomtemperature after the release of the gas, for example.

FIGS. 7A and 7B illustrate an example where current collectors arewelded to a lead electrode. As illustrated in FIG. 7A, the positiveelectrodes 503 each wrapped by the separator 507 and the negativeelectrodes 506 are alternately stacked. Then, the positive electrodecurrent collectors 501 are welded to the positive electrode leadelectrode 510, and the negative electrode current collectors 504 arewelded to the negative electrode lead electrode 511. FIG. 7B illustratesan example in which the positive electrode current collectors 501 arewelded to the positive electrode lead electrode 510. The positiveelectrode current collectors 501 are welded to the positive electrodelead electrode 510 in a welding region 512 by ultrasonic welding or thelike. The positive electrode current collector 501 includes a bentportion 513 as illustrated in FIG. 7B, and it is therefore possible torelieve stress due to external force applied after fabrication of thestorage battery 500. The reliability of the storage battery 500 can bethus increased.

In the storage battery 500 illustrated in FIG. 4 and FIGS. 5A and 5B,the positive electrode current collectors 501 in the positive electrode503 and the negative electrode current collectors 504 in the negativeelectrode 506 are welded to the positive electrode lead electrode 510and a negative electrode lead electrode 511, respectively, by ultrasonicwelding. The positive electrode current collector 501 and the negativeelectrode current collector 504 can double as terminals for electricalcontact with the outside. In that case, the positive electrode currentcollector 501 and the negative electrode current collector 504 may bearranged so that part of the positive electrode current collector 501and part of the negative electrode current collector 504 are exposed tothe outside of the exterior body 509 without using lead electrodes.

Although the positive electrode lead electrode 510 and the negativeelectrode lead electrode 511 are provided on the same side in FIG. 4 ,the positive electrode lead electrode 510 and the negative electrodelead electrode 511 may be provided on different sides as illustrated inFIG. 8 . The lead electrodes of a storage battery of one embodiment ofthe present invention can be freely positioned as described above;therefore, the degree of freedom in design is high. Accordingly, aproduct including a storage battery of one embodiment of the presentinvention can have a high degree of freedom in design. Furthermore, ayield of products each including a storage battery of one embodiment ofthe present invention can be increased.

As the exterior body 509 in the storage battery 500, for example, a filmhaving a three-layer structure in which a highly flexible metal thinfilm of aluminum, stainless steel, copper, nickel, or the like isprovided over a film formed of a material such as polyethylene,polypropylene, polycarbonate, ionomer, or polyamide, and an insulatingsynthetic resin film of a polyamide-based resin, a polyester-basedresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used.

Although the examples in FIGS. 5A and 5B each include five positiveelectrode active material layer-negative electrode active material layerpairs (the positive and negative electrode active material layers ofeach pair face each other), it is needless to say that the number ofpairs of electrode active material layers is not limited to five, andmay be more than five or less than five. In the case of using a largenumber of electrode active material layers, the storage battery can havea high capacity. In contrast, in the case of using a small number ofelectrode active material layers, the storage battery can have a smallthickness and high flexibility.

[Predoping]

Here, an example of the case where the negative electrode 506 includedin the storage battery 500 is predoped will be described. FIG. 9Aillustrates the top surface of a stack in which positive electrodes, anegative electrode, and separators are stacked, and FIG. 9B is aperspective view of FIG. 9A. A separator 507 i is stacked over thenegative electrode 506, a positive electrode 503 i for predoping isstacked over the separator 507 i, the separator 507 is stacked over thepositive electrode 503 i, and the positive electrode 503 is stacked overthe separator 507. Note that for the separator 507 i, the description ofthe separator 507 can be referred to. The positive electrode 503 iincludes a positive electrode active material layer 502 i and a positiveelectrode current collector 501 i. For the positive electrode 503 i, thepositive electrode active material layer 502 i, and the positiveelectrode current collector 501 i, the respective descriptions of thepositive electrode 503, the positive electrode active material layer502, and the positive electrode current collector 501 can be referredto. The positive electrode 503 and the positive electrode 503 i mayinclude different positive electrode active materials.

The stack illustrated in FIGS. 9A and 9B is sandwiched by a sheet 509 aserving as an exterior body as in a perspective view of FIG. 10A.

Then, three sides of the sheet 509 a are sealed with heat or the like toform the exterior body 509, so that the storage battery 500, which is athin storage battery, is fabricated, as in a top view of FIG. 10B. FIG.11A is a cross-sectional view of the storage battery along thedashed-dotted line B1-B2 in FIG. 10B.

Then, predoping is performed using the negative electrode 506 and thepositive electrode 503 i of the fabricated storage battery 500. For thepredoping, only charge or both charge and discharge can be performed,for example.

After the predoping, one side of the exterior body 509 is cut open.Then, as in a cross-sectional view of FIG. 11B, the positive electrode503 i and the separator 507 i are taken out from the opened exteriorbody 509. At this time, the separator 507 may be taken out instead ofthe separator 507 i. The separator 507 i is not necessarily taken outand may be left in the storage battery 500.

After that, the opened side of the exterior body 509 is sealed as in across-sectional view of FIG. 11C. Through the above process, predopingcan be performed.

Although the example in FIGS. 9A to 9C to FIGS. 11A to 11C includes onepositive-negative electrode active material layer pair (the positive andnegative electrode active material layers face each other), the numberof positive-negative electrode active material layer pairs whenpredoping is performed is not limited to one. The example in FIGS. 12Ato 12C includes three positive-negative electrode active material layerpairs (the positive and negative electrode active material layers ofeach pair face each other). In FIG. 12A, the positive electrode 503 i islocated between the positive electrode 503 and the negative electrode506 facing each other. First, predoping is performed using the positiveelectrode 503 i and the negative electrode 506. After that, the positiveelectrode 503 i are removed as illustrated in FIG. 12B, so that thestorage battery is obtained in which three positive-negative electrodeactive material layer pairs are included and the positive and negativeelectrode active material layers of each pair face each other asillustrated in FIG. 12C.

In the above structure, the exterior body 509 of the secondary batterycan change its form such that the smallest curvature radius is greaterthan or equal to 3 mm and less than or equal to 30 mm, preferablygreater than or equal to 3 mm and less than or equal to 10 mm. One ortwo films are used as the exterior body of the storage battery. In thecase where the storage battery has a layered structure, the storagebattery has a cross section sandwiched by two curved surfaces of thefilms when it is bent.

Description will be given of the radius of curvature of a surface withreference to FIGS. 13A to 13C. In FIG. 13A, on a plane 1701 along whicha curved surface 1700 is cut, part of a curve 1702 of the curved surface1700 is approximate to an arc of a circle, and the radius of the circleis referred to as a radius 1703 of curvature and the center of thecircle is referred to as a center 1704 of curvature. FIG. 13B is a topview of the curved surface 1700. FIG. 13C is a cross-sectional view ofthe curved surface 1700 taken along the plane 1701. When a curvedsurface is cut by a plane, the radius of curvature of a curve in a crosssection differs depending on the angle between the curved surface andthe plane or on the cut position, and the smallest radius of curvatureis defined as the radius of curvature of a surface in this specificationand the like.

In the case of bending a secondary battery in which a component 1805including electrodes, an electrolytic solution, and the like issandwiched between two films as exterior bodies, a radius 1802 ofcurvature of a film 1801 close to a center 1800 of curvature of thesecondary battery is smaller than a radius 1804 of curvature of a film1803 far from the center 1800 of curvature (FIG. 14A). When thesecondary battery is curved and has an arc-shaped cross section,compressive stress is applied to a surface of the film on the sidecloser to the center 1800 of curvature and tensile stress is applied toa surface of the film on the side farther from the center 1800 ofcurvature (FIG. 14B). However, by forming a pattern includingprojections or depressions on surfaces of the exterior bodies, theinfluence of a strain can be reduced to be acceptable even whencompressive stress and tensile stress are applied. For this reason, thesecondary battery can change its form such that the exterior body on theside closer to the center of curvature has the smallest curvature radiusgreater than or equal to 3 mm and less than or equal to 30 mm,preferably greater than or equal to 3 mm and less than or equal to 10mm.

Note that the cross-sectional shape of the secondary battery is notlimited to a simple arc shape, and the cross section can be partlyarc-shaped; for example, a shape illustrated in FIG. 14C, a wavy shapeillustrated in FIG. 14D, or an S shape can be used. When the curvedsurface of the secondary battery has a shape with a plurality of centersof curvature, the secondary battery can change its form such that acurved surface with the smallest radius of curvature among radii ofcurvature with respect to the plurality of centers of curvature, whichis a surface of the exterior body on the side closer to the center ofcurvature, has the smallest curvature radius, for example, greater thanor equal to 3 mm and less than or equal to 30 mm, preferably greaterthan or equal to 3 mm and less than or equal to 10 mm.

Next, a variety of examples of the stack of the positive electrode, thenegative electrode, and the separator will be described.

FIG. 17A illustrates an example where six positive electrodes 111 andsix negative electrodes 115 are stacked. One surface of a positiveelectrode current collector 121 included in a positive electrode 111 isprovided with a positive electrode active material layer 122. Onesurface of a negative electrode current collector 125 included in anegative electrode 115 is provided with a negative electrode activematerial layer 126.

In the structure illustrated in FIG. 17A, the positive electrodes 111and the negative electrodes 115 are stacked so that surfaces of thepositive electrodes 111 on each of which the positive electrode activematerial layer 122 is not provided are in contact with each other andthat surfaces of the negative electrodes 115 on each of which thenegative electrode active material layer 126 is not provided are incontact with each other. When the positive electrodes 111 and thenegative electrodes 115 are stacked in this manner, contact surfacesbetween metals can be formed; specifically, the surfaces of the positiveelectrodes 111 on each of which the positive electrode active materiallayer 122 is not provided can be in contact with each other, and thesurfaces of the negative electrodes 115 on each of which the negativeelectrode active material layer 126 is not provided can be in contactwith each other. The coefficient of friction of the contact surfacebetween metals can be lower than that of a contact surface between theactive material and the separator.

Therefore, when the storage battery 500 is curved, the surfaces of thepositive electrodes 111 on each of which the positive electrode activematerial layer 122 is not provided slide on each other, and the surfacesof the negative electrodes 115 on each of which the negative electrodeactive material layer 126 is not provided slide on each other; thus, thestress due to the difference between the inner diameter and the outerdiameter of a curved portion can be relieved. Here, the inner diameterof the curved portion refers to the radius of curvature of the innersurface of the curved portion in the exterior body 509 of the storagebattery 500 in the case where the storage battery 500 is curved, forexample. Therefore, the deterioration of the storage battery 500 can beinhibited. Furthermore, the storage battery 500 can have highreliability.

FIG. 17B illustrates an example of a stack of the positive electrodes111 and the negative electrodes 115 which is different from that in FIG.17A. The structure illustrated in FIG. 17B is different from that inFIG. 17A in that the positive electrode active material layers 122 areprovided on both surfaces of the positive electrode current collector121. When the positive electrode active material layers 122 are providedon both the surfaces of the positive electrode current collector 121 asillustrated in FIG. 17B, the capacity per unit volume of the storagebattery 500 can be increased.

FIG. 17C illustrates an example of a stack of the positive electrodes111 and the negative electrodes 115 which is different from that in FIG.17B. The structure illustrated in FIG. 17C is different from that inFIG. 17B in that the negative electrode active material layers 126 areprovided on both surfaces of the negative electrode current collector125. When the negative electrode active material layers 126 are providedon both the surfaces of the negative electrode current collector 125 asillustrated in FIG. 17C, the capacity per unit volume of the storagebattery 500 can be further increased.

In the structures illustrated in FIGS. 17A to 17C, the separator 123 hasa bag-like shape by which the positive electrodes 111 are surrounded;however, one embodiment of the present invention is not limited thereto.FIG. 18A illustrates an example in which the separator 123 has adifferent structure from that in FIG. 17A. The structure illustrated inFIG. 18A is different from that in FIG. 17A in that the separator 123,which is sheet-like, is provided between every pair of the positiveelectrode active material layer 122 and the negative electrode activematerial layer 126. In the structure illustrated in FIG. 18A, sixpositive electrodes 111 and six negative electrodes 115 are stacked, andsix separators 123 are provided.

FIG. 18B illustrates an example in which the separator 123 differentfrom that in FIG. 18A is provided. The structure illustrated in FIG. 18Bis different from that in FIG. 18A in that one sheet of separator 123 isfolded more than once to be interposed between every pair of thepositive electrode active material layer 122 and the negative electrodeactive material layer 126. It can be said that the structure illustratedin FIG. 18B is a structure in which the separators 123 in the respectivelayers which are illustrated in FIG. 18A are extended and connectedtogether between the layers. In the structure of FIG. 18B, six positiveelectrodes 111 and six negative electrodes 115 are stacked and thus theseparator 123 needs to be folded at least five times. The separator 123is not necessarily provided so as to be interposed between every pair ofthe positive electrode active material layer 122 and the negativeelectrode active material layer 126, and the plurality of positiveelectrodes 111 and the plurality of negative electrodes 115 may be boundtogether by extending the separator 123.

Note that the positive electrode, the negative electrode, and theseparator may be stacked as illustrated in FIGS. 19A to 19C. FIG. 19A isa cross-sectional view of a first electrode assembly 130, and FIG. 19Bis a cross-sectional view of a second electrode assembly 131. FIG. 19Cis a cross-sectional view taken along the dashed-dotted line A1-A2 inFIG. 4 . In FIG. 19C, the first electrode assembly 130, the secondelectrode assembly 131, and the separator 123 are selectivelyillustrated for the sake of clarity.

As illustrated in FIG. 19C, the storage battery 500 includes a pluralityof first electrode assemblies 130 and a plurality of second electrodeassemblies 131.

As illustrated in FIG. 19A, in each of the first electrode assemblies130, a positive electrode 111 a including the positive electrode activematerial layers 122 on both surfaces of a positive electrode currentcollector 121, the separator 123, a negative electrode 115 a includingthe negative electrode active material layers 126 on both surfaces of anegative electrode current collector 125, the separator 123, and thepositive electrode 111 a including the positive electrode activematerial layers 122 on both surfaces of the positive electrode currentcollector 121 are stacked in this order. As illustrated in FIG. 19B, ineach of the second electrode assemblies 131, the negative electrode 115a including the negative electrode active material layers 126 on bothsurfaces of the negative electrode current collector 125, the separator123, the positive electrode 111 a including the positive electrodeactive material layers 122 on both surfaces of the positive electrodecurrent collector 121, the separator 123, and the negative electrode 115a including the negative electrode active material layers 126 on bothsurfaces of the negative electrode current collector 125 are stacked inthis order.

As illustrated in FIG. 19C, the plurality of first electrode assemblies130 and the plurality of second electrode assemblies 131 are coveredwith the wound separator 123.

[Coin-Type Storage Battery]

Next, an example of a coin-type storage battery will be described as anexample of a power storage device with reference to FIGS. 15A and 15B.FIG. 15A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 15B is a cross-sectional view thereof.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305.

A negative electrode 307 includes a negative electrode current collector308 and a negative electrode active material layer 309 provided incontact with the negative electrode current collector 308.

The description of the positive electrode 503 can be referred to for thepositive electrode 304. The description of the positive electrode activematerial layer 502 can be referred to for the positive electrode activematerial layer 306. The description of the negative electrode 506 can bereferred to for the negative electrode 307. The description of thenegative electrode active material layer 505 can be referred to for thenegative electrode active material layer 309. The description of theseparator 507 can be referred to for the separator 310. The descriptionof the electrolytic solution 508 can be referred to for the electrolyticsolution.

Note that only one surface of each of the positive electrode 304 and thenegative electrode 307 used for the coin-type storage battery 300 isprovided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to an electrolytic solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel or thelike) can be used. Alternatively, the positive electrode can 301 and thenegative electrode can 302 are preferably covered with nickel, aluminum,or the like in order to prevent corrosion due to the electrolyticsolution. The positive electrode can 301 and the negative electrode can302 are electrically connected to the positive electrode 304 and thenegative electrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolytic solution. Then, asillustrated in FIG. 15B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type storage battery300 can be manufactured.

[Cylindrical Storage Battery]

Next, an example of a cylindrical storage battery will be described asan example of a power storage device with reference to FIGS. 16A and16B. As illustrated in FIG. 16A, a cylindrical storage battery 600includes a positive electrode cap (battery cap) 601 on the top surfaceand a battery can (outer can) 602 on the side surface and bottomsurface. The positive electrode cap 601 and the battery can 602 areinsulated from each other by a gasket (insulating gasket) 610.

FIG. 16B is a diagram schematically illustrating a cross section of thecylindrical storage battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with aseparator 605 interposed therebetween is provided. Although notillustrated, the battery element is wound around a center pin. One endof the battery can 602 is close and the other end thereof is open. Forthe battery can 602, a metal having a corrosion-resistant property to anelectrolytic solution, such as nickel, aluminum, or titanium, an alloyof such a metal, or an alloy of such a metal and another metal (e.g.,stainless steel or the like) can be used. Alternatively, the battery can602 is preferably covered with nickel, aluminum, or the like in order toprevent corrosion due to the electrolytic solution. Inside the batterycan 602, the battery element in which the positive electrode, thenegative electrode, and the separator are wound is provided between apair of insulating plates 608 and 609 which face each other.Furthermore, a nonaqueous electrolytic solution (not illustrated) isinjected inside the battery can 602 provided with the battery element.As the nonaqueous electrolytic solution, a nonaqueous electrolyticsolution that is similar to those of the coin-type storage battery canbe used.

The description of the positive electrode 503 can be referred to for thepositive electrode 604. The description of the negative electrode 506can be referred to for the negative electrode 606. The description ofthe method for fabricating an electrode that is described in Embodiment1 can be referred to for the positive electrode 604 and the negativeelectrode 606. Since the positive electrode and the negative electrodeof the cylindrical storage battery are wound, active materials arepreferably formed on both sides of the current collectors. A positiveelectrode terminal (positive electrode current collecting lead) 603 isconnected to the positive electrode 604, and a negative electrodeterminal (negative electrode current collecting lead) 607 is connectedto the negative electrode 606. Both the positive electrode terminal 603and the negative electrode terminal 607 can be formed using a metalmaterial such as aluminum. The positive electrode terminal 603 and thenegative electrode terminal 607 are resistance-welded to a safety valvemechanism 612 and the bottom of the battery can 602, respectively. Thesafety valve mechanism 612 is electrically connected to the positiveelectrode cap 601 through a positive temperature coefficient (PTC)element 611. The safety valve mechanism 612 cuts off electricalconnection between the positive electrode cap 601 and the positiveelectrode 604 when the internal pressure of the battery exceeds apredetermined threshold value. The PTC element 611, which serves as athermally sensitive resistor whose resistance increases as temperaturerises, limits the amount of current by increasing the resistance, inorder to prevent abnormal heat generation. Note that barium titanate(BaTiO₃)-based semiconductor ceramic can be used for the PTC element.

In the case where an electrode is wound as in the cylindrical storagebattery illustrated in FIGS. 16A and 16B, a great stress is caused atthe time of winding the electrode. In addition, an outward stress froman axis of winding is applied to the electrode all the time in the casewhere a wound body of the electrode is provided in a housing. However,the active material can be prevented from being cleaved even when such agreat stress is applied to the electrode.

Note that in this embodiment, the coin-type storage battery, thecylindrical storage battery, and the thin storage battery are given asexamples of the storage battery; however, any of storage batteries witha variety of shapes, such as a sealed storage battery and a square-typestorage battery, can be used. Furthermore, a structure in which aplurality of positive electrodes, a plurality of negative electrodes,and a plurality of separators are stacked or wound may be employed. Forexample, FIGS. 20A to 20C to FIGS. 24A and 24B illustrate examples ofother storage batteries.

Structural Example of Thin Storage Battery

FIGS. 20A to 20C and FIGS. 21A to 21C illustrate structural examples ofthin storage batteries. A wound body 993 illustrated in FIG. 20Aincludes a negative electrode 994, a positive electrode 995, and aseparator 996.

The wound body 993 is obtained by winding a sheet of a stack in whichthe negative electrode 994 overlaps with the positive electrode 995 withthe separator 996 therebetween. The wound body 993 is covered with arectangular sealed container or the like; thus, a rectangular secondarybattery is fabricated.

Note that the number of stacks each including the negative electrode994, the positive electrode 995, and the separator 996 is determined asappropriate depending on capacity and element volume which are required.The negative electrode 994 is connected to a negative electrode currentcollector (not illustrated) via one of a lead electrode 997 and a leadelectrode 998. The positive electrode 995 is connected to a positiveelectrode current collector (not illustrated) via the other of the leadelectrode 997 and the lead electrode 998.

In a storage battery 990 illustrated in FIGS. 20B and 20C, the woundbody 993 is packed in a space formed by bonding a film 981 and a film982 having a depressed portion that serve as exterior bodies bythermocompression bonding or the like. The wound body 993 includes thelead electrode 997 and the lead electrode 998, and is soaked in anelectrolytic solution inside a space surrounded by the film 981 and thefilm 982 having a depressed portion.

For the film 981 and the film 982 having a depressed portion, a metalmaterial such as aluminum or a resin material can be used, for example.With the use of a resin material for the film 981 and the film 982having a depressed portion, the film 981 and the film 982 having adepressed portion can be changed in their forms when external force isapplied; thus, a flexible storage battery can be fabricated.

Although FIGS. 20B and 20C illustrate an example where a space is formedby two films, the wound body 993 may be placed in a space formed bybending one film.

Furthermore, in fabricating a flexible power storage device, a resinmaterial or the like can be used for an exterior body and a sealedcontainer of the power storage device. In that case, a resin material orthe like is used for the exterior body and the sealed container. Notethat in the case where a resin material is used for the exterior bodyand the sealed container, a conductive material is used for a portionconnected to the outside.

For example, FIGS. 21B and 21C illustrate another example of a flexiblethin storage battery. The wound body 993 illustrated in FIG. 21A is thesame as that illustrated in FIG. 20A, and the detailed descriptionthereof is omitted.

In the storage battery 990 illustrated in FIGS. 21B and 21C, the woundbody 993 is packed in an exterior body 991. The wound body 993 includesthe lead electrode 997 and the lead electrode 998, and is soaked in anelectrolytic solution inside a space surrounded by the exterior body 991and an exterior body 992. For example, a metal material such as aluminumor a resin material can be used for the exterior bodies 991 and 992.With the use of a resin material for the exterior bodies 991 and 992,the exterior bodies 991 and 992 can be changed in their forms whenexternal force is applied; thus, a flexible thin storage battery can befabricated.

When the electrode including the active material of one embodiment ofthe present invention is used in the flexible thin storage battery, theactive material can be prevented from being cleaved even if a stresscaused by repeated bending of the thin storage battery is applied to theelectrode.

When the active material in which at least part of the cleavage plane iscovered with graphene is used for an electrode as described above, adecrease in the voltage and discharge capacity of a battery can beprevented. Accordingly, the cycle performance of the battery can beimproved.

Structural Example of Power Storage System

Structural examples of power storage systems will be described withreference to FIGS. 22A and 22B to FIGS. 24A and 24B. Here, a powerstorage system refers to, for example, a device including a powerstorage device.

FIGS. 22A and 22B are external views of a power storage system. Thepower storage system includes a circuit board 900 and a storage battery913. A label 910 is attached to the storage battery 913. As shown inFIG. 22B, the power storage system further includes a terminal 951, aterminal 952, an antenna 914, and an antenna 915.

The circuit board 900 includes terminals 911 and a circuit 912. Theterminals 911 are connected to the terminals 951 and 952, the antennas914 and 915, and the circuit 912. Note that a plurality of terminals 911serving as a control signal input terminal, a power supply terminal, andthe like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. The shape of each of the antennas 914 and 915 is not limited to acoil shape and may be a linear shape or a plate shape. Further, a planarantenna, an aperture antenna, a traveling-wave antenna, an EH antenna, amagnetic-field antenna, or a dielectric antenna may be used.Alternatively, the antenna 914 or the antenna 915 may be a flat-plateconductor. The flat-plate conductor can serve as one of conductors forelectric field coupling. That is, the antenna 914 or the antenna 915 canserve as one of two conductors of a capacitor. Thus, electric power canbe transmitted and received not only by an electromagnetic field or amagnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than that of theantenna 915. This makes it possible to increase the amount of electricpower received by the antenna 914.

The power storage system includes a layer 916 between the storagebattery 913 and the antennas 914 and 915. The layer 916 may have afunction of blocking an electromagnetic field by the storage battery913. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the power storage system is not limited tothat shown in FIGS. 22A and 22B.

For example, as shown in FIGS. 23A1 and 23A2, two opposite surfaces ofthe storage battery 913 in FIGS. 22A and 22B may be provided withrespective antennas. FIG. 23A1 is an external view showing one side ofthe opposite surfaces, and FIG. 23A2 is an external view showing theother side of the opposite surfaces. For portions similar to those inFIGS. 22A and 22B, the description of the power storage systemillustrated in FIGS. 22A and 22B can be referred to as appropriate.

As illustrated in FIG. 23A1, the antenna 914 is provided on one of theopposite surfaces of the storage battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 23A2, the antenna915 is provided on the other of the opposite surfaces of the storagebattery 913 with a layer 917 interposed therebetween. The layer 917 mayhave a function of preventing an adverse effect on an electromagneticfield by the storage battery 913. As the layer 917, for example, amagnetic body can be used.

With the above structure, both of the antennas 914 and 915 can beincreased in size.

Alternatively, as illustrated in FIGS. 23B1 and 23B2, two oppositesurfaces of the storage battery 913 in FIGS. 22A and 22B may be providedwith different types of antennas. FIG. 23B1 is an external view showingone side of the opposite surfaces, and FIG. 23B2 is an external viewshowing the other side of the opposite surfaces. For portions similar tothose in FIGS. 22A and 22B, the description of the power storage systemillustrated in FIGS. 22A and 22B can be referred to as appropriate.

As illustrated in FIG. 23B1, the antenna 914 is provided on one of theopposite surfaces of the storage battery 913 with the layer 916interposed therebetween, and as illustrated in FIG. 23B2, an antenna 918is provided on the other of the opposite surfaces of the storage battery913 with the layer 917 interposed therebetween. The antenna 918 has afunction of communicating data with an external device, for example. Anantenna with a shape that can be applied to the antennas 914 and 915,for example, can be used as the antenna 918. As a system forcommunication using the antenna 918 between the power storage system andanother device, a response method that can be used between the powerstorage system and another device, such as NFC, can be employed.

Alternatively, as illustrated in FIG. 24A, the storage battery 913 inFIGS. 22A and 22B may be provided with a display device 920. The displaydevice 920 is electrically connected to the terminal 911 via a terminal919. It is possible that the label 910 is not provided in a portionwhere the display device 920 is provided. For portions similar to thosein FIGS. 22A and 22B, the description of the power storage systemillustrated in FIGS. 22A and 22B can be referred to as appropriate.

The display device 920 can display, for example, an image showingwhether charge is being carried out, an image showing the amount ofstored power, or the like. As the display device 920, electronic paper,a liquid crystal display device, an electroluminescent (EL) displaydevice, or the like can be used. For example, the use of electronicpaper can reduce power consumption of the display device 920.

Alternatively, as illustrated in FIG. 24B, the storage battery 913illustrated in FIGS. 22A and 22B may be provided with a sensor 921. Thesensor 921 is electrically connected to the terminal 911 via a terminal922. For portions similar to those in FIGS. 22A and 22B, the descriptionof the power storage system illustrated in FIGS. 22A and 22B can bereferred to as appropriate.

As the sensor 921, a sensor that has a function of measuring, forexample, force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,electric current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared rays can be used.With the sensor 921, for example, data on an environment (e.g.,temperature) where the power storage system is placed can be determinedand stored in a memory inside the circuit 912.

The electrode of one embodiment of the present invention is used in thestorage battery and the power storage system that are described in thisembodiment. The capacity of the storage battery and the power storagesystem can thus be high. Furthermore, energy density can be high.Moreover, reliability can be high, and life can be long.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 3

In this embodiment, an example of an electronic device including aflexible power storage device will be described.

FIGS. 25A to 25G illustrate examples of electronic devices including theflexible power storage devices described in Embodiment 2. Examples ofelectronic devices each including a flexible power storage deviceinclude television devices (also referred to as televisions ortelevision receivers), monitors of computers or the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as mobile phones or mobile phonedevices), portable game machines, portable information terminals, audioreproducing devices, and large game machines such as pachinko machines.

In addition, a flexible power storage device can be incorporated along acurved inside/outside wall surface of a house or a building or a curvedinterior/exterior surface of a car.

FIG. 25A illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400includes a power storage device 7407.

FIG. 25B illustrates the mobile phone 7400 that is bent. When the wholemobile phone 7400 is bent by the external force, the power storagedevice 7407 included in the mobile phone 7400 is also bent. FIG. 25Cillustrates the bent power storage device 7407. The power storage device7407 is a thin storage battery. The power storage device 7407 is fixedin a state of being bent. Note that the power storage device 7407includes a lead electrode 7408 electrically connected to a currentcollector 7409. The current collector 7409 is, for example, copper foil,and partly alloyed with gallium; thus, adhesion between the currentcollector 7409 and an active material layer in contact with the currentcollector 7409 is improved and the power storage device 7407 can havehigh reliability even in a state of being bent.

FIG. 25D illustrates an example of a bangle display device. A portabledisplay device 7100 includes a housing 7101, a display portion 7102, anoperation button 7103, and a power storage device 7104. FIG. 25Eillustrates the bent power storage device 7104. When the display deviceis worn on a user's arm while the power storage device 7104 is bent, thehousing changes its form and the curvature of a part or the whole of thepower storage device 7104 is changed. Note that the radius of curvatureof a curve at a point refers to the radius of the circular arc that bestapproximates the curve at that point. The reciprocal of the radius ofcurvature is curvature. Specifically, a part or the whole of the housingor the main surface of the power storage device 7104 is changed in therange of radius of curvature from 40 mm to 150 mm inclusive. When theradius of curvature at the main surface of the power storage device 7104is greater than or equal to 40 mm and less than or equal to 150 mm, thereliability can be kept high.

FIG. 25F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music reproduction, Internet communication, and acomputer game.

The display surface of the display portion 7202 is curved, and imagescan be displayed on the curved display surface. In addition, the displayportion 7202 includes a touch sensor, and operation can be performed bytouching the screen with a finger, a stylus, or the like. For example,by touching an icon 7207 displayed on the display portion 7202,application can be started.

With the operation button 7205, a variety of functions such as timesetting, power on/off, on/off of wireless communication, setting andcancellation of a silent mode, and setting and cancellation of a powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely by setting the operation systemincorporated in the portable information terminal 7200.

The portable information terminal 7200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 7200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. In addition,charging via the input output terminal 7206 is possible. Note that thecharging operation may be performed by wireless power feeding withoutusing the input output terminal 7206.

The display portion 7202 of the portable information terminal 7200 isprovided with a power storage device including the electrode of oneembodiment of the present invention. For example, the power storagedevice 7104 illustrated in FIG. 25E that is in the state of being curvedcan be provided in the housing 7201. Alternatively, the power storagedevice 7104 illustrated in FIG. 25E can be provided in the band 7203such that it can be curved.

A portable information terminal 7200 preferably includes a sensor. Asthe sensor, for example a human body sensor such as a fingerprintsensor, a pulse sensor, or a temperature sensor, a touch sensor, apressure sensitive sensor, an acceleration sensor, or the like ispreferably mounted.

FIG. 25G illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the power storage deviceof one embodiment of the present invention. The display device 7300 caninclude a touch sensor in the display portion 7304 and can serve as aportable information terminal.

The display surface of the display portion 7304 is bent, and images canbe displayed on the bent display surface. A display state of the displaydevice 7300 can be changed by, for example, near field communication,which is a communication method based on an existing communicationstandard.

The display device 7300 includes an input output terminal, and data canbe directly transmitted to and received from another informationterminal via a connector. In addition, charging via the input outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input output terminal.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

In this embodiment, examples of electronic devices that can includepower storage devices will be described.

FIGS. 26A and 26B illustrate an example of a tablet terminal that can befolded in half. A tablet terminal 9600 illustrated in FIGS. 26A and 26Bincludes a housing 9630 a, a housing 9630 b, a movable portion 9640connecting the housings 9630 a and 9630 b, a display portion 9631including a display portion 9631 a and a display portion 9631 b, adisplay mode changing switch 9626, a power switch 9627, a power savingmode changing switch 9625, a fastener 9629, and an operation switch9628. FIG. 26A illustrates the tablet terminal 9600 that is opened, andFIG. 26B illustrates the tablet terminal 9600 that is closed.

The tablet terminal 9600 includes a power storage unit 9635 inside thehousings 9630 a and 9630 b. The power storage unit 9635 is providedacross the housings 9630 a and 9630 b, passing through the movableportion 9640.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that half of the area of the display portion 9631 a hasonly a display function and the other half of the area has a touch panelfunction. However, the structure of the display portion 9631 a is notlimited to this, and all the area of the display portion 9631 a may havea touch panel function. For example, all the area of the display portion9631 a can display a keyboard and serve as a touch panel while thedisplay portion 9631 b can be used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and thetouch panel region 9632 b at the same time.

The display mode changing switch 9626 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power saving mode changing switch 9625 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal 9600, which is measured with anoptical sensor incorporated in the tablet terminal 9600. In addition tothe optical sensor, other detecting devices such as sensors fordetermining inclination, such as a gyroscope or an acceleration sensor,may be incorporated in the tablet terminal.

Although the display portion 9631 a and the display portion 9631 b havethe same area in FIG. 26A, one embodiment of the present invention isnot limited to this example. The display portion 9631 a and the displayportion 9631 b may have different areas or different display quality.For example, one of the display portions 9631 a and 9631 b may displayhigher definition images than the other.

The tablet terminal is closed in FIG. 26B. The tablet terminal includesthe housing 9630, a solar cell 9633, and a charge and discharge controlcircuit 9634 including a DCDC converter 9636. The power storage unit ofone embodiment of the present invention is used as the power storageunit 9635.

The tablet terminal 9600 can be folded such that the housings 9630 a and9630 b overlap with each other when not in use. Thus, the displayportions 9631 a and 9631 b can be protected, which increases thedurability of the tablet terminal 9600. In addition, the power storageunit 9635 of one embodiment of the present invention has flexibility andcan be repeatedly bent without a significant decrease in charge anddischarge capacity. Thus, a highly reliable tablet terminal can beprovided.

The tablet terminal illustrated in FIGS. 26A and 26B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, or the time on the display portion, a touch-input function ofoperating or editing data displayed on the display portion by touchinput, a function of controlling processing by various kinds of software(programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processing portion, and the like. Note that the solarcell 9633 can be provided on one or both surfaces of the housing 9630and the power storage unit 9635 can be charged efficiently. The use of alithium-ion battery as the power storage unit 9635 brings an advantagesuch as reduction in size.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 26B will be described with reference to a blockdiagram in FIG. 26C. The solar cell 9633, the power storage unit 9635,the DCDC converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 26C, and the power storageunit 9635, the DCDC converter 9636, the converter 9637, and the switchesSW1 to SW3 correspond to the charge and discharge control circuit 9634in FIG. 26B.

First, an example of operation when electric power is generated by thesolar cell 9633 using external light will be described. The voltage ofelectric power generated by the solar cell is raised or lowered by theDCDC converter 9636 to a voltage for charging the power storage unit9635. When the display portion 9631 is operated with the electric powerfrom the solar cell 9633, the switch SW1 is turned on and the voltage ofthe electric power is raised or lowered by the converter 9637 to avoltage needed for operating the display portion 9631. When display onthe display portion 9631 is not performed, the switch SW1 is turned offand the switch SW2 is turned on, so that the power storage unit 9635 canbe charged.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, one embodiment of the present invention isnot limited to this example. The power storage unit 9635 may be chargedusing another power generation means such as a piezoelectric element ora thermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module capable of performing charging by transmitting andreceiving electric power wirelessly (without contact), or any of theother charge means used in combination.

FIG. 27 illustrates other examples of electronic devices. In FIG. 27 , adisplay device 8000 is an example of an electronic device including apower storage device 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, and the power storage device 8004.The power storage device 8004 of one embodiment of the present inventionis provided in the housing 8001. The display device 8000 can receiveelectric power from a commercial power supply. Alternatively, thedisplay device 8000 can use electric power stored in the power storagedevice 8004. Thus, the display device 8000 can be operated with the useof the power storage device 8004 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 27 , an installation lighting device 8100 is an example of anelectronic device including a power storage device 8103 of oneembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, and the power storagedevice 8103. Although FIG. 27 illustrates the case where the powerstorage device 8103 is provided in a ceiling 8104 on which the housing8101 and the light source 8102 are installed, the power storage device8103 may be provided in the housing 8101. The lighting device 8100 canreceive electric power from a commercial power supply. Alternatively,the lighting device 8100 can use electric power stored in the powerstorage device 8103. Thus, the lighting device 8100 can be operated withthe use of power storage device 8103 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 27 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the power storage device of one embodiment of the presentinvention can be used in a tabletop lighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 27 , an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device including apower storage device 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, and the power storage device 8203. Although FIG. 27illustrates the case where the power storage device 8203 is provided inthe indoor unit 8200, the power storage device 8203 may be provided inthe outdoor unit 8204. Alternatively, the power storage devices 8203 maybe provided in both the indoor unit 8200 and the outdoor unit 8204. Theair conditioner can receive electric power from a commercial powersupply. Alternatively, the air conditioner can use electric power storedin the power storage device 8203. Particularly in the case where thepower storage devices 8203 are provided in both the indoor unit 8200 andthe outdoor unit 8204, the air conditioner can be operated with the useof the power storage device 8203 of one embodiment of the presentinvention as an uninterruptible power supply even when electric powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 27 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 27 , an electric refrigerator-freezer 8300 is an example of anelectronic device including a power storage device 8304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, and the power storagedevice 8304. The power storage device 8304 is provided in the housing8301 in FIG. 27 . The electric refrigerator-freezer 8300 can receiveelectric power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 8300 can use electric power stored in thepower storage device 8304. Thus, the electric refrigerator-freezer 8300can be operated with the use of the power storage device 8304 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

Note that among the electronic devices described above, a high-frequencyheating apparatus such as a microwave oven and an electronic device suchas an electric rice cooker require high power in a short time. Thetripping of a breaker of a commercial power supply in use of anelectronic device can be prevented by using the power storage device ofone embodiment of the present invention as an auxiliary power supply forsupplying electric power which cannot be supplied enough by a commercialpower supply.

In addition, in a time period when electronic devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the power storage device, whereby the usage rate ofelectric power can be reduced in a time period when the electronicdevices are used. For example, in the case of the electricrefrigerator-freezer 8300, electric power can be stored in the powerstorage device 8304 in night time when the temperature is low and thedoor for a refrigerator 8302 and the door for a freezer 8303 are notoften opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 8302 and the doorfor a freezer 8303 are frequently opened and closed, the power storagedevice 8304 is used as an auxiliary power supply; thus, the usage rateof electric power in daytime can be reduced.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 5

In this embodiment, examples of vehicles using power storage deviceswill be described.

The use of power storage devices in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HEVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHEVs).

FIGS. 28A and 28B each illustrate an example of a vehicle using oneembodiment of the present invention. An automobile 8400 illustrated inFIG. 28A is an electric vehicle that runs on the power of an electricmotor. Alternatively, the automobile 8400 is a hybrid electric vehiclecapable of driving appropriately using either an electric motor or anengine. One embodiment of the present invention can provide ahigh-mileage vehicle. The automobile 8400 includes the power storagedevice. The power storage device is used not only for driving theelectric motor 8406, but also for supplying electric power to alight-emitting device such as a headlight 8401 or a room light (notillustrated).

The power storage device can also supply electric power to a displaydevice of a speedometer, a tachometer, or the like included in theautomobile 8400. Furthermore, the power storage device can supplyelectric power to a semiconductor device included in the automobile8400, such as a navigation system.

FIG. 28B illustrates an automobile 8500 including the power storagedevice. The automobile 8500 can be charged when the power storage deviceis supplied with electric power through external charging equipment by aplug-in system, a contactless power feeding system, or the like. In FIG.28B, a power storage device 8024 included in the automobile 8500 ischarged with the use of a ground-based charging apparatus 8021 through acable 8022. In charging, a given method such as CHAdeMO (registeredtrademark) or Combined Charging System may be employed as a chargingmethod, the standard of a connector, or the like as appropriate. Theground-based charging apparatus 8021 may be a charging station providedin a commerce facility or a power source in a house. For example, withthe use of a plug-in technique, the power storage device 8024 includedin the automobile 8500 can be charged by being supplied with electricpower from outside. The charging can be performed by converting ACelectric power into DC electric power through a converter such as anAC-DC converter.

Furthermore, although not illustrated, the vehicle may include a powerreceiving device so that it can be charged by being supplied withelectric power from an above-ground power transmitting device in acontactless manner. In the case of the contactless power feeding system,by fitting a power transmitting device in a road or an exterior wall,charging can be performed not only when the electric vehicle is stoppedbut also when driven. In addition, the contactless power feeding systemmay be utilized to perform transmission and reception of electric powerbetween vehicles. Furthermore, a solar cell may be provided in theexterior of the automobile to charge the power storage device when theautomobile stops or moves. To supply electric power in such acontactless manner, an electromagnetic induction method or a magneticresonance method can be used.

According to one embodiment of the present invention, the power storagedevice can have improved cycle performance and reliability. Furthermore,according to one embodiment of the present invention, the power storagedevice itself can be made more compact and lightweight as a result ofimproved characteristics of the power storage device. The compact andlightweight power storage device contributes to a reduction in theweight of a vehicle, and thus increases the driving distance.Furthermore, the power storage device included in the vehicle can beused as a power source for supplying electric power to products otherthan the vehicle. In such a case, the use of a commercial power sourcecan be avoided at peak time of electric power demand.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 6

A battery control unit (battery management unit: BMU) that can be usedin combination with battery cells each including the materials describedin the above embodiment and transistors that are suitable for a circuitincluded in the battery control unit will be described with reference toFIG. 29 to FIG. 35 . In this embodiment, in particular, a batterycontrol unit of a power storage device including battery cells connectedin series will be described.

When the plurality of battery cells connected in series are repeatedlycharged and discharged, there occurs variations in charge and dischargecharacteristics among the battery cells, which causes variations incapacity (output voltage) among the battery cells. The dischargecapacity of all the plurality of battery cells connected in seriesdepends on the capacity of the battery cell that is low. The variationsin capacity among the battery cells reduce the discharge capacity of allthe battery cells. Furthermore, when charge is performed based on thecapacity of the battery cell that is low, the battery cells might beundercharged. In contrast, when charge is performed based on thecapacity of the battery cell that is high, the battery cells might beovercharged.

Thus, the battery control unit of the power storage device including thebattery cells connected in series has a function of reducing variationsin capacity among the battery cells, which cause an undercharge and anovercharge. Examples of a circuit configuration for reducing variationsin capacity among battery cells include a resistive type, a capacitivetype, and an inductive type, and a circuit configuration that can reducevariations in capacity among battery cells using transistors with a lowoff-state current will be explained here as an example.

A transistor including an oxide semiconductor in its channel formationregion (an OS transistor) is preferably used as the transistor with alow off-state current. When an OS transistor with a low off-statecurrent is used in the circuit of the battery control unit of the powerstorage device, the amount of charge that leaks from a battery can bereduced, and reduction in capacity with the lapse of time can besuppressed.

As the oxide semiconductor used in the channel formation region, anIn-M-Zn oxide (M is Ga, Sn, Y, Zr, La, Ce, or Nd) is used. In the casewhere the atomic ratio of the metal elements of a target for forming anoxide semiconductor film is In:M:Zn=x₁:y₁:z₁, x₁/y₁ is preferablygreater than or equal to ⅓ and less than or equal to 6, more preferablygreater than or equal to 1 and less than or equal to 6, and z₁/y₁ ispreferably greater than or equal to ⅓ and less than or equal to 6, morepreferably greater than or equal to 1 and less than or equal to 6. Notethat when z₁/y₁ is greater than or equal to 1 and less than or equal to6, a CAAC-OS film as the oxide semiconductor film is easily formed.

Here, the details of the CAAC-OS film will be described.

A CAAC-OS film is one of oxide semiconductor films having a plurality ofc-axis aligned crystal parts.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OSfilm, which is obtained using a TEM, a plurality of crystal parts can beobserved. However, in the high-resolution TEM image, a boundary betweencrystal parts, that is, a grain boundary is not clearly observed. Thus,in the CAAC-OS film, a reduction in electron mobility due to the grainboundary is less likely to occur.

According to the high-resolution cross-sectional TEM image of theCAAC-OS film observed in the direction substantially parallel to thesample surface, metal atoms are arranged in a layered manner in thecrystal parts. Each metal atom layer reflects unevenness of a surfaceover which the CAAC-OS film is formed (hereinafter, a surface over whichthe CAAC-OS film is formed is referred to as a formation surface) or thetop surface of the CAAC-OS film, and is arranged parallel to theformation surface or the top surface of the CAAC-OS film.

On the other hand, according to the plan high-resolution TEM image ofthe CAAC-OS film observed in the direction substantially perpendicularto the sample surface, metal atoms are arranged in a triangular orhexagonal arrangement in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

For example, when the structure of a CAAC-OS including an InGaZnO₄crystal is analyzed by an out-of-plane method using an X-ray diffraction(XRD) apparatus, a peak may appear at a diffraction angle (2θ) of around31°. This peak is derived from the (009) plane of the InGaZnO₄ crystal,which indicates that crystals in the CAAC-OS film have c-axis alignment,and that the c-axes are aligned in the direction substantiallyperpendicular to the formation surface or the top surface of the CAAC-OSfilm.

Note that in analysis of the CAAC-OS film with an InGaZnO₄ crystal by anout-of-plane method, another peak may appear when 2θ is around 36°, inaddition to the peak at 2θ of around 31°. The peak at 2θ of around 36°indicates that a crystal having no c-axis alignment is included in partof the CAAC-OS film. It is preferable that in the CAAC-OS film, a peakappear when 2θ is around 31° and that a peak not appear when 2θ isaround 36°.

The CAAC-OS film is an oxide semiconductor film with low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element (specifically,silicon or the like) having higher strength of bonding to oxygen than ametal element included in an oxide semiconductor film extracts oxygenfrom the oxide semiconductor film, which results in disorder of theatomic arrangement and reduced crystallinity of the oxide semiconductorfilm. Furthermore, a heavy metal such as iron or nickel, argon, carbondioxide, or the like has a large atomic radius (molecular radius), andthus disturbs the atomic arrangement of the oxide semiconductor film andcauses a decrease in crystallinity when it is contained in the oxidesemiconductor film. Note that the impurity contained in the oxidesemiconductor might serve as a carrier trap or a carrier generationsource.

The CAAC-OS film is an oxide semiconductor having a low density ofdefect states. In some cases, oxygen vacancies in the oxidesemiconductor film serve as carrier traps or serve as carrier generationsources when hydrogen is captured therein, for example.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas a “highly purified intrinsic” or “substantially intrinsic” state. Ahighly purified intrinsic or substantially intrinsic oxide semiconductorhas few carrier generation sources, and thus can have a low carrierdensity. Therefore, a transistor including the oxide semiconductor filmrarely has negative threshold voltage (is rarely normally on). Thehighly purified intrinsic or substantially intrinsic oxide semiconductorfilm has few carrier traps. Accordingly, the transistor including theoxide semiconductor film has little variation in electricalcharacteristics and high reliability. Charge trapped by the carriertraps in the oxide semiconductor film takes a long time to be releasedand might behave like fixed charge. Thus, the transistor including theoxide semiconductor film having high impurity concentration and a highdensity of defect states has unstable electrical characteristics in somecases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Since the OS transistor has a wider band gap than a transistor includingsilicon in its channel formation region (a Si transistor), dielectricbreakdown is unlikely to occur when a high voltage is applied. Althougha voltage of several hundreds of volts is generated when battery cellsare connected in series, the above-described OS transistor is suitablefor a circuit of a battery control unit which is used for such batterycells in the power storage device.

FIG. 29 is an example of a block diagram of the power storage device. Apower storage device BT00 illustrated in FIG. 29 includes a terminalpair BT01, a terminal pair BT02, a switching control circuit BT03, aswitching circuit BT04, a switching circuit BT05, a voltagetransformation control circuit BT06, a voltage transformer circuit BT07,and a battery portion BT08 including a plurality of battery cells BT09connected in series.

In the power storage device BT00 illustrated in FIG. 29 , a portionincluding the terminal pair BT01, the terminal pair BT02, the switchingcontrol circuit BT03, the switching circuit BT04, the switching circuitBT05, the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07 can be referred to as a battery control unit.

The switching control circuit BT03 controls operations of the switchingcircuits BT04 and BT05. Specifically, the switching control circuit BT03selects battery cells to be discharged (a discharge battery cell group)and battery cells to be charged (a charge battery cell group) inaccordance with voltage measured for every battery cell BT09.

Furthermore, the switching control circuit BT03 outputs a control signalSi and a control signal S2 on the basis of the selected dischargebattery cell group and the selected charge battery cell group. Thecontrol signal S1 is output to the switching circuit BT04. The controlsignal S1 controls the switching circuit BT04 so that the terminal pairBT01 and the discharge battery cell group are connected. In addition,the control signal S2 is output to the switching circuit BT05. Thecontrol signal S2 controls the switching circuit BT05 so that theterminal pair BT02 and the charge battery cell group are connected.

The switching control circuit BT03 generates the control signal Si andthe control signal S2 on the basis of the connection relation of theswitching circuit BT04, the switching circuit BT05, and the voltagetransformer circuit BT07 so that terminals having the same polarity ofthe terminal pair BT01 and the discharge battery cell group areconnected with each other, or terminals having the same polarity of theterminal pair BT02 and the charge battery cell group are connected witheach other.

The operations of the switching control circuit BT03 will be describedin detail.

First, the switching control circuit BT03 measures the voltage of eachof the plurality of battery cells BT09. Then, the switching controlcircuit BT03 determines that the battery cell BT09 having a voltagehigher than a predetermined threshold value is a high-voltage batterycell (high-voltage cell) and that the battery cell BT09 having a voltagelower than the predetermined threshold value is a low-voltage batterycell (low-voltage cell), for example.

As a method to determine whether a battery cell is a high-voltage cellor a low-voltage cell, any of various methods can be employed. Forexample, the switching control circuit BT03 may determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell on thebasis of the voltage of the battery cell BT09 having the highest voltageor the lowest voltage among the plurality of battery cells BT09. In thiscase, the switching control circuit BT03 can determine whether eachbattery cell BT09 is a high-voltage cell or a low-voltage cell by, forexample, determining whether or not the ratio of the voltage of eachbattery cell BT09 to the reference voltage is the predetermined value ormore. Then, the switching control circuit BT03 determines a chargebattery cell group and a discharge battery cell group on the basis ofthe determination result.

Note that high-voltage cells and low-voltage cells are mixed in variousstates in the plurality of battery cells BT09. For example, theswitching control circuit BT03 selects a portion having the largestnumber of high-voltage cells connected in series as the dischargebattery cell group of mixed high-voltage cells and low-voltage cells.Furthermore, the switching control circuit BT03 selects a portion havingthe largest number of low-voltage cells connected in series as thecharge battery cell group. In addition, the switching control circuitBT03 may preferentially select the battery cells BT09 which are almostovercharged or over-discharged as the discharge battery cell group orthe charge battery cell group.

Here, operation examples of the switching control circuit BT03 in thisembodiment will be described with reference to FIGS. 30A to 30C. FIGS.30A to 30C illustrate the operation examples of the switching controlcircuit BT03. Note that FIGS. 30A to 30C each illustrate the case wherefour battery cells BT09 are connected in series as an example forconvenience of explanation.

FIG. 30A shows the case where the relation of voltages Va, Vb, Vc, andVd is Va=Vb=Vc>Vd where the voltages Va, Vb, Vc, and Vd are the voltagesof a battery cell a, a battery cell b, a battery cell c, and a batterycell d, respectively. That is, a series of three high-voltage cells a toc and one low-voltage cell d are connected in series. In this case, theswitching control circuit BT03 selects the series of three high-voltagecells a to c as the discharge battery cell group. In addition, theswitching control circuit BT03 selects the low-voltage cell d as thecharge battery cell group.

Next, FIG. 30B shows the case where the relation of the voltages isVc>Va=Vb>>Vd. That is, a series of two low-voltage cells a and b, onehigh-voltage cell c, and one low-voltage cell d which is almostover-discharged are connected in series. In this case, the switchingcontrol circuit BT03 selects the high-voltage cell c as the dischargebattery cell group. Since the low-voltage cell d is almostover-discharged, the switching control circuit BT03 preferentiallyselects the low-voltage cell d as the charge battery cell group insteadof the series of two low-voltage cells a and b.

Lastly, FIG. 30C shows the case where the relation of the voltages isVa>Vb=Vc=Vd. That is, one high-voltage cell a and a series of threelow-voltage cells b to d are connected in series. In this case, theswitching control circuit BT03 selects the high-voltage cell a as thedischarge battery cell group. In addition, the switching control circuitBT03 selects the series of three low-voltage cells b to d as the chargebattery cell group.

On the basis of the determination result shown in the examples of FIGS.30A to 30C, the switching control circuit BT03 outputs the controlsignal S1 and the control signal S2 to the switching circuit BT04 andthe switching circuit BT05, respectively. Information showing thedischarge battery cell group, which is the connection destination of theswitching circuit BT04, is set in the control signal S1. Informationshowing the charge battery cell group, which is the connectiondestination of the switching circuit BT05 is set in the control signalS2.

The above is the detailed description of the operations of the switchingcontrol circuit BT03.

The switching circuit BT04 sets the connection destination of theterminal pair BT01 at the discharge battery cell group selected by theswitching control circuit BT03, in response to the control signal S1output from the switching control circuit BT03.

The terminal pair BT01 includes a pair of terminals A1 and A2. Theswitching circuit BT04 connects one of the pair of terminals A1 and A2to a positive electrode terminal of the battery cell BT09 positioned onthe most upstream side (on the high potential side) of the dischargebattery cell group, and the other to a negative electrode terminal ofthe battery cell BT09 positioned on the most downstream side (on the lowpotential side) of the discharge battery cell group. Note that theswitching circuit BT04 can recognize the position of the dischargebattery cell group on the basis of the information set in the controlsignal S1.

The switching circuit BT05 sets the connection destination of theterminal pair BT02 at the charge battery cell group selected by theswitching control circuit BT03, in response to the control signal S2output from the switching control circuit BT03.

The terminal pair BT02 includes a pair of terminals B1 and B2. Theswitching circuit BT05 sets the connection destination of the terminalpair BT02 by connecting one of the pair of terminals B1 and B2 to apositive electrode terminal of the battery cell BT09 positioned on themost upstream side (on the high potential side) of the charge batterycell group, and the other to a negative electrode terminal of thebattery cell BT09 positioned on the most downstream side (on the lowpotential side) of the charge battery cell group. Note that theswitching circuit BT05 can recognize the position of the charge batterycell group on the basis of the information set in the control signal S2.

FIG. 31 and FIG. 32 are circuit diagrams showing configuration examplesof the switching circuits BT04 and BT05.

In FIG. 31 , the switching circuit BT04 includes a plurality oftransistors BT10, a bus BT11, and a bus BT12. The bus BT11 is connectedto the terminal A1. The bus BT12 is connected to the terminal A2.Sources or drains of the plurality of transistors BT10 are connectedalternately to the bus BT11 and the bus BT12. The sources or drainswhich are not connected to the bus BT11 and the bus BT12 of theplurality of transistors BT10 are each connected between two adjacentbattery cells BT09.

The source or drain of the transistor BT10 on the most upstream side ofthe plurality of transistors BT10 is connected to the positive electrodeterminal of the battery cell BT09 on the most upstream side of thebattery portion BT08. The source or drain of the transistor BT10 on themost downstream side of the plurality of transistors BT10 is connectedto the negative electrode terminal of the battery cell BT09 on the mostdownstream side of the battery portion BT08.

The switching circuit BT04 connects the discharge battery cell group tothe terminal pair BT01 by bringing one of the plurality of transistorsBT10 which are connected to the bus BT11 and one of the plurality oftransistors BT10 which are connected to the bus BT12 into an on state inresponse to the control signal Si supplied to gates of the plurality oftransistors BT10. Accordingly, the positive electrode terminal of thebattery cell BT09 on the most upstream side of the discharge batterycell group is connected to one of the pair of terminals A1 and A2. Inaddition, the negative electrode terminal of the battery cell BT09 onthe most downstream side of the discharge battery cell group isconnected to the other of the pair of terminals A1 and A2 (i.e., aterminal which is not connected to the positive electrode terminal).

An OS transistor is preferably used as the transistor BT10. Since theoff-state current of the OS transistor is low, the amount of charge thatleaks from the battery cell which does not belong to the dischargebattery cell group can be reduced, and reduction in capacity with thelapse of time can be suppressed. In addition, dielectric breakdown isunlikely to occur in the OS transistor when a high voltage is applied.Therefore, the battery cell BT09 and the terminal pair BT01, which areconnected to the transistor BT10 in an off state, can be insulated fromeach other even when the output voltage of the discharge battery cellgroup is high.

In FIG. 31 , the switching circuit BT05 includes a plurality oftransistors BT13, a current control switch BT14, a bus BT15, and a busBT16. The bus BT15 and the bus BT16 are provided between the pluralityof transistors BT13 and the current control switch BT14. Sources ordrains of the plurality of transistors BT13 are connected alternately tothe bus BT15 and the bus BT16. The sources or drains which are notconnected to the bus BT15 and the bus BT16 of the plurality oftransistors BT13 are each connected between two adjacent battery cellsBT09.

The source or drain of the transistor BT13 on the most upstream side ofthe plurality of transistors BT13 is connected to the positive electrodeterminal of the battery cell BT09 on the most upstream side of thebattery portion BT08. The source or a drain of the transistor BT13 onthe most downstream side of the plurality of transistors BT13 isconnected to the negative electrode terminal of the battery cell BT09 onthe most downstream side of the battery portion BT08.

An OS transistor is preferably used as the transistors BT13 like thetransistors BT10. Since the off-state current of the OS transistor islow, the amount of charge that leaks from the battery cells which do notbelong to the charge battery cell group can be reduced, and reduction incapacity with the lapse of time can be suppressed. In addition,dielectric breakdown is unlikely to occur in the OS transistor when ahigh voltage is applied. Therefore, the battery cell BT09 and theterminal pair BT02, which are connected to the transistor BT13 in an offstate, can be insulated from each other even when a voltage for chargingthe charge battery cell group is high.

The current control switch BT14 includes a switch pair BT17 and a switchpair BT18. Terminals on one end of the switch pair BT17 are connected tothe terminal B1. Terminals on the other end of the switch pair BT17branch off from two switches. One switch is connected to the bus BT15,and the other switch is connected to the bus BT16. Terminals on one endof the switch pair BT18 are connected to the terminal B2. Terminals onthe other end of the switch pair BT18 extend from two switches. Oneswitch is connected to the bus BT15, and the other switch is connectedto the bus BT16.

OS transistors are preferably used for the switches included in theswitch pair BT17 and the switch pair BT18 like the transistors BT10 andBT13.

The switching circuit BT05 connects the charge battery cell group andthe terminal pair BT02 by controlling the combination of on and offstates of the transistors BT13 and the current control switch BT14 inresponse to the control signal S2.

For example, the switching circuit BT05 connects the charge battery cellgroup and the terminal pair BT02 in the following manner.

The switching circuit BT05 brings a transistor BT13 connected to thepositive electrode terminal of the battery cell BT09 on the mostupstream side of the charge battery cell group into an on state inresponse to the control signal S2 supplied to gates of the plurality oftransistors BT13. In addition, the switching circuit BT05 brings atransistor BT13 connected to the negative electrode terminal of thebattery cell BT09 on the most downstream side of the charge battery cellgroup into an on state in response to the control signal S2 supplied tothe gates of the plurality of transistors BT13.

The polarities of voltages applied to the terminal pair BT02 can vary inaccordance with the configurations of the voltage transformer circuitBT07 and the discharge battery cell group connected to the terminal pairBT01. In order to supply a current in the direction for charging thecharge battery cell group, terminals with the same polarity of theterminal pair BT02 and the charge battery cell group are required to beconnected to each other. In view of this, the current control switchBT14 is controlled by the control signal S2 so that the connectiondestination of the switch pair BT17 and that of the switch pair BT18 arechanged in accordance with the polarities of the voltages applied to theterminal pair BT02.

The state where voltages are applied to the terminal pair BT02 so as tomake the terminal B1 a positive electrode and the terminal B2 a negativeelectrode will be described as an example. Here, in the case where thebattery cell BT09 positioned on the most downstream side of the batteryportion BT08 is in the charge battery cell group, the switch pair BT17is controlled to be connected to the positive electrode terminal of thebattery cell BT09 in response to the control signal S2. That is, theswitch of the switch pair BT17 connected to the bus BT16 is turned on,and the switch of the switch pair BT17 connected to the bus BT15 isturned off. In contrast, the switch pair BT18 is controlled to beconnected to the negative electrode terminal of the battery cell BT09positioned on the most downstream side of the battery portion BT08 inresponse to the control signal S2. That is, the switch of the switchpair BT18 connected to the bus BT15 is turned on, and the switch of theswitch pair BT18 connected to the bus BT16 is turned off. In thismanner, terminals with the same polarity of the terminal pair BT02 andthe charge battery cell group are connected to each other. In addition,the current which flows from the terminal pair BT02 is controlled to besupplied in a direction so as to charge the charge battery cell group.

In addition, instead of the switching circuit BT05, the switchingcircuit BT04 may include the current control switch BT14.

FIG. 32 is a circuit diagram illustrating configuration examples of theswitching circuit BT04 and the switching circuit BT05 which aredifferent from those of FIG. 31 .

In FIG. 32 , the switching circuit BT04 includes a plurality oftransistor pairs BT21, a bus BT24, and a bus BT25. The bus BT24 isconnected to the terminal A1. The bus BT25 is connected to the terminalA2. Terminals on one end of each of the plurality of transistor pairsBT21 branch off from a transistor BT22 and a transistor BT23. Sources ordrains of the transistors BT22 are connected to the bus BT24. Sources ordrains of the transistors BT23 are connected to the bus BT25. Inaddition, terminals on the other end of each of the plurality oftransistor pairs are connected between two adjacent battery cells BT09.The terminals on the other end of the transistor pair BT21 on the mostupstream side of the plurality of transistor pairs BT21 are connected tothe positive electrode terminal of the battery cell BT09 on the mostupstream side of the battery portion BT08. The terminals on the otherend of the transistor pair BT21 on the most downstream side of theplurality of transistor pairs BT21 are connected to a negative electrodeterminal of the battery cell BT09 on the most downstream side of thebattery portion BT08.

The switching circuit BT04 switches the connection destination of thetransistor pair BT21 to one of the terminal A1 and the terminal A2 byturning on or off the transistors BT22 and BT23 in response to thecontrol signal S1. Specifically, when the transistor BT22 is turned on,the transistor BT23 is turned off, so that the connection destination ofthe transistor pair BT21 is the terminal A1. On the other hand, when thetransistor BT23 is turned on, the transistor BT22 is turned off, so thatthe connection destination of the transistor pair BT21 is the terminalA2. Which of the transistors BT22 and BT23 is turned on is determined bythe control signal S1.

Two transistor pairs BT21 are used to connect the terminal pair BT01 andthe discharge battery cell group. Specifically, the connectiondestinations of the two transistor pairs BT21 are determined on thebasis of the control signal S1, and the discharge battery cell group andthe terminal pair BT01 are connected. The connection destinations of thetwo transistor pairs BT21 are controlled by the control signal Si sothat one of the connection destinations is the terminal A1 and the otheris the terminal A2.

The switching circuit BT05 includes a plurality of transistor pairsBT31, a bus BT34, and a bus BT35. The bus BT34 is connected to theterminal B1. The bus BT35 is connected to the terminal B2. Terminals onone end of each of the plurality of transistor pairs BT31 branch offfrom a transistor BT32 and a transistor BT33. One terminal extendingfrom the transistor BT32 is connected to the bus BT34. The otherterminal extending from the transistor BT33 is connected to the busBT35. Terminals on the other end of each of the plurality of transistorpairs BT31 are connected between two adjacent battery cells BT09. Theterminal on the other end of the transistor pair BT31 on the mostupstream side of the plurality of transistor pairs BT31 is connected tothe positive electrode terminal of the battery cell BT09 on the mostupstream side of the battery portion BT08. The terminal on the other endof the transistor pair BT31 on the most downstream side of the pluralityof transistor pairs BT31 is connected to the negative electrode terminalof the battery cell BT09 on the most downstream side of the batteryportion BT08.

The switching circuit BT05 switches the connection destination of thetransistor pair BT31 to one of the terminal B1 and the terminal B2 byturning on or off the transistors BT32 and BT33 in response to thecontrol signal S2. Specifically, when the transistor BT32 is turned on,the transistor BT33 is turned off, so that the connection destination ofthe transistor pair BT31 is the terminal B1. On the other hand, when thetransistor BT33 is turned on, the transistor BT32 is turned off, so thatthe connection destination of the transistor pair BT31 is the terminalB2. Which of the transistors BT32 and BT33 is turned on is determined bythe control signal S2.

Two transistor pairs BT31 are used to connect the terminal pair BT02 andthe charge battery cell group. Specifically, the connection destinationsof the two transistor pairs BT31 are determined on the basis of thecontrol signal S2, and the charge battery cell group and the terminalpair BT02 are connected. The connection destinations of the twotransistor pairs BT31 are controlled by the control signal S2 so thatone of the connection destinations is the terminal B1 and the other isthe terminal B2.

The connection destinations of the two transistor pairs BT31 aredetermined by the polarities of the voltages applied to the terminalpair BT02. Specifically, in the case where voltages which make theterminal B1 a positive electrode and the terminal B2 a negativeelectrode are applied to the terminal pair BT02, the transistor pairBT31 on the upstream side is controlled by the control signal S2 so thatthe transistor BT32 is turned on and the transistor BT33 is turned off.In contrast, the transistor pair BT31 on the downstream side iscontrolled by the control signal S2 so that the transistor BT33 isturned on and the transistor BT32 is turned off. In the case wherevoltages which make the terminal B1 a negative electrode and theterminal B2 a positive electrode are applied to the terminal pair BT02,the transistor pair BT31 on the upstream side is controlled by thecontrol signal S2 so that the transistor BT33 is turned on and thetransistor BT32 is turned off. In contrast, the transistor pair BT31 onthe downstream side is controlled by the control signal S2 so that thetransistor BT32 is turned on and the transistor BT33 is turned off. Inthis manner, terminals with the same polarity of the terminal pair BT02and the charge battery cell group are connected to each other. Inaddition, the current which flows from the terminal pair BT02 iscontrolled to be supplied in the direction for charging the chargebattery cell group.

The voltage transformation control circuit BT06 controls the operationof the voltage transformer circuit BT07. The voltage transformationcontrol circuit BT06 generates a voltage transformation signal S3 forcontrolling the operation of the voltage transformer circuit BT07 on thebasis of the number of the battery cells BT09 included in the dischargebattery cell group and the number of the battery cells BT09 included inthe charge battery cell group and outputs the voltage transformationsignal S3 to the voltage transformer circuit BT07.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, it is necessary to prevent a charging voltage whichis too high from being applied to the charge battery cell group. Thus,the voltage transformation control circuit BT06 outputs the voltagetransformation signal S3 for controlling the voltage transformer circuitBT07 so that a discharging voltage (Vdis) is lowered within a rangewhere the charge battery cell group can be charged.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is less than or equal to that included inthe charge battery cell group, a charging voltage necessary for chargingthe charge battery cell group needs to be ensured. Therefore, thevoltage transformation control circuit BT06 outputs the voltagetransformation signal S3 for controlling the voltage transformer circuitBT07 so that the discharging voltage (Vdis) is raised within a rangewhere a charging voltage which is too high is not applied to the chargebattery cell group.

The voltage value of the charging voltage which is too high isdetermined in the light of product specifications and the like of thebattery cell BT09 used in the battery portion BT08. The voltage which israised or lowered by the voltage transformer circuit BT07 is applied asa charging voltage (Vcha) to the terminal pair BT02.

Here, operation examples of the voltage transformation control circuitBT06 in this embodiment will be described with reference to FIGS. 33A to33C. FIG. 33A to 33C are conceptual diagrams for explaining theoperation examples of the voltage transformation control circuits BT06for the discharge battery cell groups and the charge battery cell groupsdescribed in FIGS. 30A to 30C. FIGS. 33A to 33C each illustrate abattery control unit BT41. The battery control unit BT41 includes theterminal pair BT01, the terminal pair BT02, the switching controlcircuit BT03, the switching circuit BT04, the switching circuit BT05,the voltage transformation control circuit BT06, and the voltagetransformer circuit BT07.

In an example illustrated in FIG. 33A, the series of three high-voltagecells a to c and one low-voltage cell d are connected in series asdescribed in FIG. 30A. In this case, as described using FIG. 30A, theswitching control circuit BT03 determines the high-voltage cells a to cas the discharge battery cell group, and determines the low-voltage celld as the charge battery cell group. The voltage transformation controlcircuit BT06 calculates a conversion ratio N for converting thedischarging voltage (Vdis) into the charging voltage (Vcha) based on theratio of the number of the battery cells BT09 included in the chargebattery cell group to the number of the battery cells BT09 included inthe discharge battery cell group.

In the case where the number of the battery cells BT09 included in thedischarge battery cell group is larger than that included in the chargebattery cell group, when a discharging voltage is applied to theterminal pair BT02 without transforming the voltage, an overvoltage maybe applied to the battery cells BT09 included in the charge battery cellgroup through the terminal pair BT02. Thus, in the case of FIG. 33A, itis necessary that a charging voltage (Vcha) applied to the terminal pairBT02 be lower than the discharging voltage. In addition, in order tocharge the charge battery cell group, it is necessary that the chargingvoltage be higher than the total voltage of the battery cells BT09included in the charge battery cell group. Thus, the voltagetransformation control circuit BT06 sets the conversion ratio N forraising or lowering voltage larger than the ratio of the number of thebattery cells BT09 included in the charge battery cell group to thenumber of the battery cells BT09 included in the discharge battery cellgroup.

Thus, the voltage transformation control circuit BT06 preferably setsthe conversion ratio N for raising or lowering voltage larger than theratio of the number of the battery cells BT09 included in the chargebattery cell group to the number of the battery cells BT09 included inthe discharge battery cell group by about 1% to 10%. Here, the chargingvoltage is made higher than the voltage of the charge battery cellgroup, but the charging voltage is equal to the voltage of the chargebattery cell group in reality. Note that the voltage transformationcontrol circuit BT06 feeds a current for charging the charge batterycell group in accordance with the conversion ratio N for raising orlowering voltage in order to make the voltage of the charge battery cellgroup equal to the charging voltage. The value of the current is set bythe voltage transformation control circuit BT06.

In the example illustrated in FIG. 33A, since the number of the batterycells BT09 included in the discharge battery cell group is three and thenumber of the battery cells BT09 included in the charge battery cellgroup is one, the voltage transformation control circuit BT06 calculatesa value which is slightly larger than ⅓ as the conversion ratio N forraising or lowering voltage. Then, the voltage transformation controlcircuit BT06 outputs the voltage transformation signal S3, which lowersthe discharging voltage in accordance with the conversion ratio N forraising or lowering voltage and converts the voltage into a chargingvoltage, to the voltage transformer circuit BT07. The voltagetransformer circuit BT07 applies the charging voltage which is obtainedby transformation in response to the voltage transformation signal S3 tothe terminal pair BT02. Then, the battery cells BT09 included in thecharge battery cell group are charged with the charging voltage appliedto the terminal pair BT02.

In each of examples illustrated in FIGS. 33B and 33C, the conversionratio N for raising or lowering voltage is calculated in a mannersimilar to that of FIG. 33A. In each of the examples illustrated inFIGS. 33B and 33C, since the number of the battery cells BT09 includedin the discharge battery cell group is less than or equal to the numberof the battery cells BT09 included in the charge battery cell group, theconversion ratio N for raising or lowering voltage is 1 or more.Therefore, in this case, the voltage transformation control circuit BT06outputs the voltage transformation signal S3 for raising the dischargingvoltage and converting the voltage into the charging voltage.

The voltage transformer circuit BT07 converts the discharging voltageapplied to the terminal pair BT01 into a charging voltage in response tothe voltage transformation signal S3. The voltage transformer circuitBT07 applies the charging voltage to the terminal pair BT02. Here, thevoltage transformer circuit BT07 electrically insulates the terminalpair BT01 from the terminal pair BT02. Accordingly, the voltagetransformer circuit BT07 prevents a short circuit due to a differencebetween the absolute voltage of the negative electrode terminal of thebattery cell BT09 on the most downstream side of the discharge batterycell group and the absolute voltage of the negative electrode terminalof the battery cell BT09 on the most downstream side of the chargebattery cell group. Furthermore, the voltage transformer circuit BT07converts the discharging voltage, which is the total voltage of thedischarge battery cell group, into the charging voltage in response tothe voltage transformation signal S3 as described above.

An insulated direct current (DC)-DC converter or the like can be used inthe voltage transformer circuit BT07. In that case, the voltagetransformation control circuit BT06 controls the charging voltageconverted by the voltage transformer circuit BT07 by outputting a signalfor controlling the on/off ratio (the duty ratio) of the insulated DC-DCconverter as the voltage transformation signal S3.

Examples of the insulated DC-DC converter include a flyback converter, aforward converter, a ringing choke converter (RCC), a push-pullconverter, a half-bridge converter, and a full-bridge converter, and asuitable converter is selected in accordance with the value of theintended output voltage.

The configuration of the voltage transformer circuit BT07 including theinsulated DC-DC converter is illustrated in FIG. 34 . An insulated DC-DCconverter BT51 includes a switch portion BT52 and a transformer BT53.The switch portion BT52 is a switch for switching on/off of theinsulated DC-DC converter, and a metal oxide semiconductor field-effecttransistor (MOSFET), a bipolar transistor, or the like is used as theswitch portion BT52. The switch portion BT52 periodically turns on andoff the insulated DC-DC converter BT51 in response to the voltagetransformation signal S3 for controlling the on/off ratio which isoutput from the voltage transformation control circuit BT06. The switchportion BT52 can have any of various structures in accordance with thetype of the insulated DC-DC converter which is used. The transformerBT53 converts the discharging voltage applied from the terminal pairBT01 into the charging voltage. In detail, the transformer BT53 operatesin conjunction with the on/off state of the switch portion BT52 andconverts the discharging voltage into the charging voltage in accordancewith the on/off ratio. As the time during which the switch portion BT52is on becomes longer in its switching period, the charging voltage isincreased. On the other hand, as the time during which the switchportion BT52 is on becomes shorter in its switching period, the chargingvoltage is decreased. In the case where the insulated DC-DC converter isused, the terminal pair BT01 and the terminal pair BT02 can be insulatedfrom each other inside the transformer BT53.

A flow of operations of the power storage device BT00 in this embodimentwill be described with reference to FIG. 35 . FIG. 35 is a flow chartshowing the flow of the operations of the power storage device BT00.

First, the power storage device BT00 obtains a voltage measured for eachof the plurality of battery cells BT09 (step S101). Then, the powerstorage device BT00 determines whether or not the condition for startingthe operation of reducing variations in voltage of the plurality ofbattery cells BT09 is satisfied (step S102). For example, the conditionthat the difference between the maximum value and the minimum value ofthe voltage measured for each of the plurality of battery cells BT09 ishigher than or equal to the predetermined threshold value can be used.In the case where the condition is not satisfied (step S102: NO), thepower storage device BT00 does not perform the following operationbecause voltages of the battery cells BT09 are well balanced. Incontrast, in the case where the condition is satisfied (step S102: YES),the power storage device BT00 performs the operation of reducingvariations in the voltage of the battery cells BT09. In this operation,the power storage device BT00 determines whether each battery cell BT09is a high voltage cell or a low voltage cell on the basis of themeasured voltage of each cell (step S103). Then, the power storagedevice BT00 determines a discharge battery cell group and a chargebattery cell group on the basis of the determination result (step S104).In addition, the power storage device BT00 generates the control signalS1 for setting the connection destination of the terminal pair BT01 tothe determined discharge battery cell group, and the control signal S2for setting the connection destination of the terminal pair BT02 to thedetermined charge battery cell group (step S105). The power storagedevice BT00 outputs the generated control signals S1 and S2 to theswitching circuit BT04 and the switching circuit BT05, respectively.Then, the switching circuit BT04 connects the terminal pair BT01 and thedischarge battery cell group, and the switching circuit BT05 connectsthe terminal pair BT02 and the discharge battery cell group (step S106).The power storage device BT00 generates the voltage transformationsignal S3 based on the number of the battery cells BT09 included in thedischarge battery cell group and the number of the battery cells BT09included in the charge battery cell group (step S107). Then, the powerstorage device BT00 converts, in response to the voltage transformationsignal S3, the discharging voltage applied to the terminal pair BT01into a charging voltage and applies the charging voltage to the terminalpair BT02 (step S108). In this way, charge of the discharge battery cellgroup is transferred to the charge battery cell group.

Although the plurality of steps are shown in order in the flow chart ofFIG. 35 , the order of performing the steps is not limited to the order.

According to the above embodiment, when charge is transferred from thedischarge battery cell group to the charge battery cell group, astructure where charge from the discharge battery cell group istemporarily stored, and the stored charge is sent to the charge batterycell group is unnecessary, unlike in the a capacitive type circuit.Accordingly, the charge transfer efficiency per unit time can beincreased. In addition, the switching circuit BT04 and the switchingcircuit BT05 determine which battery cell in the discharge battery cellgroup and the charge battery cell group to be connected to the voltagetransformer circuit.

Furthermore, the voltage transformer circuit BT07 converts thedischarging voltage applied to the terminal pair BT01 into the chargingvoltage based on the number of the battery cells BT09 included in thedischarge battery cell group and the number of the battery cells BT09included in the charge battery cell group, and applies the chargingvoltage to the terminal pair BT02. Thus, charge can be transferredwithout any problems regardless of how the battery cells BT09 areselected as the discharge battery cell group and the charge battery cellgroup.

Furthermore, the use of OS transistors as the transistor BT10 and thetransistor BT13 can reduce the amount of charge that leaks from thebattery cells BT09 not belonging to the charge battery cell group or thedischarge battery cell group. Accordingly, a decrease in capacity of thebattery cells BT09 which do not contribute to charging or dischargingcan be suppressed. In addition, the variations in characteristics of theOS transistor due to heat are smaller than those of a Si transistor.Accordingly, even when the temperature of the battery cells BT09 isincreased, an operation such as turning on or off the transistors inresponse to the control signals Si and S2 can be performed normally.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Example 1

In this example, electrolytic solutions were subjected to cyclicvoltammetry (CV) measurement.

Four kinds of electrolytic solutions, Electrolytic Solution A-1,Electrolytic Solution A-2, Electrolytic Solution A-3, and ElectrolyticSolution A-4, were prepared. In each of the four kinds of electrolyticsolutions, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide(EMI-FSA) was used as a solvent. In each of Electrolytic Solutions A-2,A-3, and A-4, lithium bis(fluorosulfonyl)amide (LiFSA) was used as anelectrolyte. In Electrolytic Solution A-1, an electrolyte was not used.The electrolyte concentrations are 1 mol/L in Electrolytic Solution A-2,0.1 mol/L in Electrolytic Solution A-3, and 0.01 mol/L in ElectrolyticSolution A-4.

Lithium, copper, and platinum were used as a reference electrode, aworking electrode, and a counter electrode, respectively. The area ofthe working electrode was 0.02 cm². In an electrolytic solution for thereference electrode, N-methyl-N-propylpiperidiniumbis(trifluoromethanesulfonyl)amide (PP13-TFSA) was used as a solvent andlithium bis(trifluoromethanesulfonyl)amide (LiTFSA) was used as anelectrolyte. The electrolyte concentration in the electrolytic solutionwas 0.4 mol/L.

The scanning rate was 50 mV/s, and the voltage range was from 3 V to−0.5 V. FIGS. 36A and 36B and FIGS. 37A and 37B show CV measurementresults of Electrolytic Solutions A-1, A-2, A-3, and A-4, respectively.The horizontal axis represents potential (vs. Li/Li⁺), and the verticalaxis represents current density.

For Electrolytic Solution A-1, which does not use a lithium salt, areduction current was observed at approximately 0.7 V, which suggests areduction reaction of EMI-FSA. For Electrolytic Solution A-2, areduction current at approximately 0.7 V observed in the case ofElectrolytic Solution A-1 was inhibited and a reduction current and anoxidation current at approximately 0 V, which suggest lithium depositionand lithium dissolution, respectively, were observed. These resultsimply that in Electrolytic Solution A-2, lithium ions form an electricdouble layer on a surface of the electrode, resulting in inhibition of areduction reaction of either or both of EMI cations and FSA anions.

For Electrolytic Solution A-3, a noticeable reduction current was notobserved until approximately 0 V, which suggests inhibition ofdecomposition of EMI; however, an oxidation current, which suggestslithium deposition, was not observed. In contrast, for ElectrolyticSolution A-4, a reduction current was observed at a potential higherthan 0 V, which suggests that a reduction reaction of either or both ofEMI cations and FSA anions occurs.

FIGS. 38A and 38B show results of second and third cycles of CVmeasurement of Electrolytic Solution A-4. The waveforms of the secondand third cycles of CV measurement of Electrolytic Solution A-4 werealmost the same as the waveform of CV measurement of ElectrolyticSolution A-1, which does not use a lithium salt. Presumably, inElectrolytic Solution A-4, the concentration of a lithium salt is toolow to allow sufficient formation of an electric double layer on theelectrode surface, so that reduction reactions of EMI cations and thelike cannot be inhibited.

The CV measurement was performed on Electrolytic Solution A-4 at a sweeprate of 0.5 mV/s within the voltage range from 3 V to −0.3 V, and it wasfound that a reduction reaction is inhibited. FIG. 39 shows the results.

Next, Electrolytic Solutions A-5 and A-6 were prepared. In each ofElectrolytic Solutions A-5 and A-6, EMI-TFSA was used as a solvent andLiTFSA was used as an electrolyte. The electrolyte concentrations were 1mol/L in Electrolytic Solution A-5 and 2 mol/L in Electrolytic SolutionA-6.

FIG. 40 shows results of CV measurement of Electrolytic Solution A-5 ata sweep rate of 0.5 mV/s within the voltage range from 3 V to −0.15 V.FIG. 41A shows results of CV measurement of Electrolytic Solution A-6 ata sweep rate of 0.5 mV/s within the voltage range from 3 V to 0 V. FIG.41B shows results of CV measurement of Electrolytic Solution A-6 at asweep rate of 0.1 mV/s within the voltage range from 3 V to 0 V.Electrolytic Solution A-6, in which the concentration of a lithium saltis 2 mol/L, shows a peak suggesting lithium dissolution, which indicatesthat a formed electric double layer might inhibit reduction reactions ofEMI cations and the like.

Example 2

In this example, storage batteries were fabricated and the charge anddischarge cycles thereof were measured.

First, Electrolytic Solutions A-7 and AC-1 were prepared. InElectrolytic Solution A-7, BMI-FSA was used as a solvent and LiFSA wasused as an electrolyte. The electrolyte concentration in ElectrolyticSolution A-7 was 1.5 mol/L. In Electrolytic Solution AC-1, which is acomparative electrolytic solution, a solvent obtained by mixing EC andDEC at a volume ratio of EC:DEC=3:7 was used and LiPF₆ was used as anelectrolyte. The electrolyte concentration in Electrolytic Solution AC-1was 1 mol/L.

[Fabrication of Electrodes]

Next, Negative Electrode E1 and Positive Electrodes E2 and E3 wereprepared.

The compounding and fabricating method of Negative Electrode E1 will bedescribed. Carbon-coated SiO was used as an active material, polyimidewas used as a binder, and acetylene black was used as a conductiveadditive. In a slurry for fabricating the electrode, the compoundingratio of SiO to acetylene black and polyimide was 80:5:15 (wt %).

First, acetylene black and NMP serving as a solvent were mixed with amixer, so that a first mixture was obtained.

Next, the active material was added to the first mixture, so that asecond mixture was obtained.

After that, an NMP solution of polyimide was added to the second mixtureand mixing was performed with a mixer. Through the above steps, theslurry was formed.

The formed slurry was applied to one surface of a 10-μm-thickstainless-steel current collector. The application was performed with acontinuous coater at a coating speed of 0.15 m/min. After that, dryingwas performed using a drying furnace at 40° C. for 20 minutes.

Then, the fabricated electrode was subjected to heat treatment forimidization. The heat treatment was performed with a muffle furnace at400° C. in a nitrogen atmosphere for 5 hours. Through the above steps,the negative electrode was fabricated.

Next, Positive Electrodes E2 and E3 were prepared.

The compounding and fabricating conditions of Positive Electrode E2 willbe described. LiCoO₂ with an average particle size of 10 μm was used asan active material, PVdF was used as a binder, and acetylene black wasused as a conductive additive. In a slurry for fabricating theelectrode, the compounding ratio of LiCoO₂ to acetylene black and PVdFwas 90:5:5 (wt %).

The active material, the binder, and the conductive additive were mixedto form a slurry. The slurry was applied to one surface of a 20-μm-thickaluminum current collector. Then, the solvent was volatilized. Afterthat, the positive electrode active material layer was pressed. Throughthe above steps, Positive Electrode E2 was fabricated.

Next, the compounding and manufacturing conditions of Positive ElectrodeE3 will be described. LiFePO₄ with a specific surface area of 15.6 m²/gwas used as an active material, PVdF was used as a binder, and acetyleneblack was used as a conductive additive. In a slurry for fabricating theelectrode, the compounding ratio of LiFePO₄ to acetylene black and PVdFwas 85:8:7 (wt %).

First, acetylene black and PVdF serving as a binder were mixed with amixer, so that a first mixture was obtained.

Next, the active material was added to the first mixture, so that asecond mixture was obtained.

After that, a solvent NMP was added to the second mixture and mixing wasperformed with a mixer. Through the above steps, the slurry was formed.

Then, mixing was performed with a large-sized mixer.

Then, the formed slurry was applied to one surface of a 20-μm-thickaluminum current collector subjected to undercoating in advance. Theapplication was performed with a continuous coater at a coating speed of0.2 m/min. Then, the solvent was volatilized with a drying furnace. Thesolvent volatilization in the drying furnace was performed at 70° C. for7.5 minutes and then further performed at 90° C. for 7.5 minutes.

Subsequently, the positive electrode active material layer was pressedby a roll press method so as to be consolidated. Through the abovesteps, Positive Electrode E3 was fabricated.

[Fabrication of Storage Batteries]

Next, Negative Electrode E1, Positive Electrode E2, and PositiveElectrode E3 were cut, and lead electrodes were bonded to respective tabregions by welding. The area of Negative Electrode E1 was 23.8 cm², andthe area of each of Positive Electrodes E2 and E3 was 20.5 cm².

Then, Negative Electrode E1 and Positive Electrode E2 were provided soas to face each other with a first separator therebetween, a secondseparator was provided over Positive Electrode E2, and PositiveElectrode E3 was provided over the second separator. The secondseparator, Positive Electrode E2, and the first separator are locatedcloser to Positive Electrode E3 in this order between Negative ElectrodeE1 and Positive Electrode E3. As each separator, TF40 made withcellulose having a thickness of 40 μm was used.

As a sheet for forming an exterior body, an aluminum sheet both surfacesof which are covered with a resin was prepared. The stack of NegativeElectrode E1, the first separator, Positive Electrode E2, the secondseparator, and Positive Electrode E3 was wrapped with the sheet foldedin half.

Next, two of three sides of the sheet folded in half were sealed withheat, so that the exterior body was formed. End portions of the leadelectrodes bonded to the respective electrodes were located outside thesheet. After that, drying was performed at 80° C.

Then, an electrolytic solution was injected from a side that was notsealed, in a reduced-pressure atmosphere of −60 kPa or less. The amountof the injected electrolytic solution was approximately 0.6 ml. Astorage battery using Electrolytic Solution A-7 is referred to asStorage Battery BA, and a comparative storage battery using ElectrolyticSolution AC-1, which is a comparative electrolytic solution, is referredto as Storage Battery BC.

The loadings of Negative Electrode E1, Positive Electrode E2, andPositive Electrode E3 used in Storage Battery BA were 2.0 mg/cm², 20mg/cm², and 9.3 mg/cm², respectively. The densities of NegativeElectrode E1, Positive Electrode E2, and Positive Electrode E3 inStorage Battery BA were 0.55 g/cc, approximately 2.2 g/cc, and 1.0 g/cc,respectively.

The loadings of Negative Electrode E1, Positive Electrode E2, andPositive Electrode E3 used in Storage Battery BC were 2.0 mg/cm², 20mg/cm², and 9.4 mg/cm², respectively. The densities of NegativeElectrode E1, Positive Electrode E2, and Positive Electrode E3 inStorage Battery BC were 0.63 g/cc, approximately 2.1 g/cc, and 1.1 g/cc,respectively. Note that loading means the weight of an active materialper unit area.

Then, the side which was not sealed was sealed with heat. Through theabove steps, each storage battery was fabricated.

[Predoping]

Constant current charge was performed at 25° C. at a current density of17.9 mA/g (corresponding to approximately 0.01 C with respect to thenegative electrode active material capacity) using Positive Electrode E2as a positive electrode and Negative Electrode E1 as a negativeelectrode up to 600 mAh/g. A 0.5-hour break was taken after the charge.

After the charge, one side of the exterior body was cut. Then, PositiveElectrode E2 and the second separator were taken out from the side thatwas cut.

Next, 0.3 ml of Electrolytic Solution A-7 was injected into StorageBattery BA, and 0.3 ml of Electrolytic Solution AC-1 was injected intoStorage Battery BC.

After that, the side cut open was sealed with heat.

[Aging]

Next, an aging step was performed. First, constant current charge wasperformed at 25° C. using Positive Electrode E3 as a positive electrodeand Negative Electrode E1 as a negative electrode. The chargingconditions was as follows: the current density was 1.7 mA/g(corresponding to approximately 0.01 C with respect to the positiveelectrode active material capacity), and the upper voltage limit was 3.2V. A 2-hour break was taken after the charge.

After the charge, one side of the exterior body was cut, anddegasification was performed. After that, the side cut open was sealedwith heat.

Then, constant current charge was performed under the conditions thatthe current density was 8.5 mA/g (corresponding to approximately 0.05 C)and the upper voltage limit was 4 V. After that, constant currentdischarge was performed under the conditions that the current densitywas 34 mA/g (corresponding to approximately 0.2 C) and the lower voltagelimit was 2 V. After each of the charge and discharge, a 2-hour breakwas taken.

Then, two cycles of constant current charge and discharge were performedat a current density of 34 mA/g. FIGS. 42A and 42B show charge anddischarge curves of the two cycles. In each of the graphs, the solidlines show the initial charge and discharge, and dotted lines show thesecond charge and discharge. FIG. 42A shows charge and discharge cyclesof Storage Battery BC, and FIG. 42B shows charge and discharge cycles ofStorage Battery BA. The upper charging voltage limit was 4 V, and thelower discharging voltage limit was 2 V. After each of the charge anddischarge, a 2-hour break was taken. Through the above steps, aging wasperformed.

[Charge and Discharge Cycles]

The charge and discharge cycle performances of Storage Batteries BA andBC subjected to the aging were evaluated. Charge and discharge wereperformed at a constant current at a current density of 51 mA/g(corresponding to approximately 0.3 C). The upper charging voltage limitwas 4 V, and the lower discharging voltage limit was 2 V. After each ofthe charge and discharge, a 0.5-hour break was taken. FIG. 43 shows thecharge and discharge cycle performances. The horizontal axis representsthe number of cycles, and the vertical axis represents dischargecapacity. The discharge capacity in the 500th cycle of Storage BatteryBC was as low as approximately 60% of the initial discharge capacity,whereas the discharge capacity in the 500th cycle of Storage Battery BAwas over 80%, which indicates excellent charge and discharge cycleperformance of Storage Battery BA.

Example 3

Surfaces of the negative electrodes of Storage Batteries BA and BCfabricated in Example 2 were subjected to XPS analysis after charge anddischarge cycles.

After charge and discharge cycles, Storage Batteries BA and BC weredisassembled. After the disassembly, deuterated acetonitrile was addedto be mixed with the electrolytic solution left in each of the storagebatteries, and the mixed solution was extracted with a pipette. Then,the negative electrode was immersed in DMC, which was put in a petridish, to be washed. DMC was renewed twice, and washing was performedthree times in total. After that, the solvent was volatilized in areduced-pressure atmosphere.

Next, the negative electrodes of Storage Batteries BA and BC weresubjected to XPS analysis. Table 1 shows the proportions of C, O, F, S,Li, P, N, Al, and Si (atomic %).

TABLE 1 C O F S Li P N Al Si Storage Battery BC 32 31 7.9 — 28 0.3 0.3 —0.7 Storage Battery BA 31 38 1.4 3.8 24 — 2.1 — —

FIG. 44A shows C1s spectra. FIG. 44B shows O1s spectra. FIG. 45A showsF1s spectra. FIG. 45B shows S2p spectra. FIG. 46 shows Li1s spectra.

A prominent difference was observed between peaks of the S2p spectrum.The SO_(x) peak of the negative electrode of Storage Battery BA is threeor more times the metal-S peak.

Example 4

Cross sections of the negative electrodes of Storage Batteries BA and BCfabricated in Example 2 were observed after charged and dischargecycles.

After charge and discharge cycles, Storage Batteries BA and BC weredisassembled. After the disassembly, deuterated acetonitrile was addedto be mixed with the electrolytic solution left in each of the storagebatteries, and the mixed solution was extracted with a pipette. Then,the negative electrode was immersed in DMC, which was put in a petridish, to be washed. DMC was renewed twice, and washing was performedthree times in total. After that, the solvent was volatilized in areduced-pressure atmosphere.

Next, to observe the cross sections of the negative electrodes, thenegative electrodes were sliced by FIB. Then, the cross sections of thenegative electrodes were observed with a TEM. FIGS. 47 and 48 show theobserved negative electrodes of Storage Batteries BC and BA,respectively.

Then, the cross sections of the negative electrodes were observed with aSTEM, and STEM-EDX line analyses were performed as elementary analysesof the following six elements: carbon, oxygen, fluorine, silicon,phosphorus, and sulfur. FIG. 49A shows a STEM image of a portion ofStorage Battery BC on which line analysis was performed. FIG. 49B andFIG. 51A each show spectra obtained by the line analysis. FIG. 50A showsa STEM image of a portion of Storage Battery BA on which line analysiswas performed. FIG. 50B and FIG. 51B each show spectra obtained by theline analysis. Note that the vertical axis in FIG. 49B and FIG. 50Brepresents signal intensity, and the vertical axis in FIG. 51A and FIG.51B represents element content (atomic %).

In FIG. 49B and FIG. 51A, a region from the surface to a depth ofapproximately 0.2 μm is referred to as a third region, a region from adepth of approximately 0.2 μm to a depth of approximately 0.45 μm isreferred to as a second region, and a region deeper than a depth ofapproximately 0.45 μm is referred to as a first region. In the firstregion, constituents of SiO, which is a negative electrode activematerial, appear to be mainly detected. In the third region,constituents of a coating film formed by decomposition of theelectrolytic solution appear to be mainly detected. The second region ispresumably a mixed region of the negative electrode active material andthe coating film. As shown in the EDX analysis results of FIG. 49B, theintensity of carbon is 10 or more times that of silicon in the thirdregion, whereas the intensity of carbon is approximately 1 time that ofsilicon in the second region and the intensity of carbon is one sixth orless of that of silicon in the first region. As shown in the EDXanalysis results of FIG. 51A, the number of carbon atoms is 20 or moretimes that of silicon in the third region, whereas the number of carbonatoms is approximately 4 times that of silicon in the second region andthe number of carbon atoms is approximately 1 time that of silicon inthe first region.

In FIG. 50B and FIG. 51B, a region from the surface to a depth ofapproximately 0.1 μm is referred to as a third region, a region from adepth of approximately 0.1 μm to a depth of approximately 0.2 μm isreferred to as a second region, and a region deeper than a depth ofapproximately 0.2 μm is referred to as a first region. As shown in theEDX analysis results of FIG. 50B, the intensity of carbon is 10 or moretimes that of silicon in the third region, whereas the intensity ofcarbon is approximately 1 time that of silicon in the second region andthe intensity of carbon is one sixth or less of that of silicon in thefirst region. As shown in the EDX analysis results of FIG. 51B, thenumber of carbon atoms is 20 or more times that of silicon in the thirdregion, whereas the number of carbon atoms is approximately 3 or moretimes that of silicon in the second region and the number of carbonatoms is approximately 1 time that of silicon in the first region.

Next, EELS line analyses were performed as elementary analyses. FIG. 52Ashows a SEM image of a portion of Storage Battery BC on which lineanalysis was performed. FIG. 52B shows spectra obtained by the lineanalysis. FIG. 53A shows a SEM image of a portion of Storage Battery BAon which line analysis was performed. FIG. 53B shows spectra obtained bythe line analysis. Note that the vertical axis represents signalintensity.

In FIG. 52B, a region from the surface to a depth of approximately 0.2 mis referred to as a third region, a region from a depth of approximately0.2 μm to a depth of approximately 0.45 μm is referred to as a secondregion, and a region deeper than a depth of approximately 0.45 μm isreferred to as a first region. As shown in the EELS analysis results ofFIG. 52B, the intensity of carbon is 10 or more times that of silicon inthe third region, whereas the intensity of carbon is approximately halfof that of silicon in the second region and the intensity of carbon isone tenth or less of that of silicon in the first region.

In FIG. 53B, a region from the surface to a depth of approximately 0.1μm is referred to as a third region, a region from a depth ofapproximately 0.1 μm to a depth of approximately 0.2 μm is referred toas a second region, and a region deeper than a depth of approximately0.2 μm is referred to as a first region. As shown in the EELS analysisresults of FIG. 53B, the intensity of carbon is 10 or more times that ofsilicon in the third region, whereas the intensity of carbon isapproximately one tenth or less of that of silicon in the first region.

FIG. 47 to FIGS. 53A and 53B show that the second region in the negativeelectrode of Storage Battery BA is thinner than that in the negativeelectrode of Storage Battery BC, which suggests that the surface of theactive material is prevented from being cracked and pulverized.

This application is based on Japanese Patent Application serial no.2016-016346 filed with Japan Patent Office on Jan. 29, 2016, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A storage battery comprising: a positiveelectrode; a negative electrode; and an electrolytic solution, whereinthe negative electrode includes a first region, a second region being incontact with a surface of the first region, and a third region being incontact with a surface of the second region, wherein each of the secondregion and the third region has a layered shape, wherein a thickness ofthe second region is larger than or equal to 10 nm and smaller than orequal to 500 nm, wherein a thickness of the third region is larger thanor equal to 10 nm and smaller than or equal to 1000 nm, wherein each ofthe first region, the second region, and the third region comprisescarbon and a first element, wherein the first element is one selectedfrom the group consisting of silicon, tin, gallium, aluminum, germanium,lead, antimony, bismuth, silver, zinc, cadmium, and indium, wherein anatomic ratio of carbon to the first element included in the first regionis x₁:y₁, wherein an atomic ratio of carbon to the first elementincluded in the second region is x₂:y₂, wherein an atomic ratio ofcarbon to the first element included in the third region is x₃:y₃,wherein x₂/y₂ is larger than x₁/y₁, wherein x₃/y₃ is larger than x₂/y₂,wherein the electrolytic solution includes a first cation and an ionicliquid, wherein the ionic liquid contains a second cation and an anion,wherein the first cation is one selected from the group consisting of alithium ion, a sodium ion, a calcium ion, and a magnesium ion, whereinthe second cation is one of an imidazolium cation and a tertiarysulfonium cation, and wherein the anion is one selected from the groupconsisting of a monovalent amide-based anion, a monovalent methide-basedanion, a fluorosulfonate anion (SO₃F⁻), a fluoroalkylsulfonate anion, atetrafluoroborate anion (BF₄ ⁻), a fluoroalkylborate anion, ahexafluorophosphate anion (PF₆ ⁻), and a fluoroalkylphosphate anion. 2.An electronic device comprising the storage battery according toclaim
 1. 3. An electronic device comprising: the storage batteryaccording to claim 1; and a display device.
 4. An electronic devicecomprising: the storage battery according to claim 1; and aninput-output terminal having a function of performing wirelesscommunication.
 5. A storage battery comprising: a positive electrode; anegative electrode; and an electrolytic solution, wherein the negativeelectrode includes a first region, a second region being in contact witha surface of the first region, and a third region being in contact witha surface of the second region, wherein each of the second region andthe third region has a layered shape, wherein a thickness of the secondregion is larger than or equal to 10 nm and smaller than or equal to 500nm, wherein a thickness of the third region is larger than or equal to10 nm and smaller than or equal to 1000 nm, wherein each of the firstregion, the second region, and the third region comprises carbon and afirst element, wherein the first element is one selected from the groupconsisting of silicon, tin, gallium, aluminum, germanium, lead,antimony, bismuth, silver, zinc, cadmium, and indium, wherein an atomicratio of carbon to the first element included in the first region isx₁:y₁, wherein an atomic ratio of carbon to the first element includedin the second region is x₂:y₂, wherein an atomic ratio of carbon to thefirst element included in the third region is x₃:y₃, wherein x₁/y₁ issmaller than or equal to 3, wherein x₂/y₂ is larger than or equal to 0.1and smaller than 10, wherein x₃/y₃ is larger than or equal to 5, whereinx₂/y₂ is larger than x₁/y₁, wherein x₃/y₃ is larger than x₂/y₂, whereinthe electrolytic solution includes a first cation and an ionic liquid,wherein the ionic liquid contains a second cation and an anion, whereinthe first cation is one selected from the group consisting of a lithiumion, a sodium ion, a calcium ion, and a magnesium ion, wherein thesecond cation is one of an imidazolium cation and a tertiary sulfoniumcation, and wherein the anion is one selected from the group consistingof a monovalent amide-based anion, a monovalent methide-based anion, afluorosulfonate anion (SO₃F⁻), a fluoroalkylsulfonate anion, atetrafluoroborate anion (BF₄ ⁻), a fluoroalkylborate anion, ahexafluorophosphate anion (PF₆ ⁻), and a fluoroalkylphosphate anion. 6.An electronic device comprising the storage battery according to claim5.
 7. An electronic device comprising: the storage battery according toclaim 5; and a display device.
 8. An electronic device comprising: thestorage battery according to claim 5; and an input-output terminalhaving a function of performing wireless communication.